We are finishing the unfinshed 25% to R-20 and are wondering what kind of an increase that will give (and if we need to do the other 15% of the house this year to qualify for the grant).

Is there some sort of calculation based on the square footages at what R value to figure out the overall R value?

I would think the same outfit that did your energy audit could come up with the projected averages, even if not real world.

here are all sorts of seals available for windows to reduce the infiltration loss.
Even older storms can benefit from weather stripping (added or original).

The formula for computing R-value is: (hr x degrees F x sqft)/Btu. It is important to note the "per sqft" part of the formula.

Heat travels in three ways, conduction, convection, and radiation. R-value numbers are based on the thermal resistance to conducted heat loss in a square foot of the material to be tested and R-value "measurements" do not take into account either radiation or convection losses (or gains).

Imagine an exterior wall with a couple of windows, 2x6 studs - 16" OC, electrical outlets, and fiberglass insulation. Many folks would automatically say that this wall has an insulating value of R-19 based on 6-inches thickness of fiberglass plus wall sheathing plus drywall.

While it is possible to "add" R-value numbers - siding + sheathing + insulation + drywall - the final figure would be accurate only between studs and only if there were no "breaks" in any of the materials listed. If a stud, or a window, or even an electrical outlet were involved, then the R-value numbers could become meaningless. And accurate only in the sense of measuring the "ideal" of a portion of the wall with regards to resistance to conductive heat flow.

In the real world, R-value is most useful because it is easy to "see" the difference between a material that is "R-10" and a material that is "R-20".

But also in the real world, R-value is not really a very realistic measurement when applied to an entire structure - unless the structure was subject to an energy audit and the stated R-value was computed using U-value formulas which were then converted into R-value for easier comprehension.

The formula for computing U-value is: Btu / (hr x degrees F x sqft)
And as previously mentioned, the formula for computing R-value is: (hr x degrees F x sqft) / Btu

To illustrate a simple example, imagine that we have one sqft of insulation (type and thickness not important for this example). We have a temp of 70º on one side of the material and 0º on the other; we can now find the U-value if we know how much energy it took to keep the 70º side at 70º for one hour.

If it takes 3.68 Btu’s to keep the 70º constant for that hour, then we have 3.68 / (1 hr x 70º x 1 sqft) = a U-value of .05257.

Using the same scenario, the calculation for R-value would be (1 hr x 70º x 1 sqft) / 3.68 or an R-value of 19.02.

Since 19.02 = 1/.05257 or .05257 = 1/19.02 - R-value and U-value have an inverse or reciprocal relationship - but they have entirely different meanings - and while you cannot accurately calculate the R-value of a structure using the R-value formula, you can do so using U-value formula and then converting to an R-vlaue-based number.

Not exactly. You compute a weighted factor for the wall including the studs and insulation by area.
The same is done for windows, especially if existing construction when data is rarely available.

There are any number of references that give values for commonly built walls (brick, masonry, frame, insulation value, etc.), ceilings, etc.
The ASHRAE Fundamentals book has an entire chapter that covers most construction methods for the past ~50 years.
Infiltration is tougher to get a handle on, so a blower door test is a useful measurement. It is possible to convert the 'air changes per hour' into a volume number and then determine a leakage loss value.
You can even use previous heating bills to determine usage and then adjust for the weather conditions that occurred at the time.
I often use this as a check on calculations. The experiment has been performed if the place has heating (or cooling). All you need to do is extract the results.

It can be rather expensive to have a detailed Manual J (heat loss/gain) calculation performed on an older house, and just driving purchased SW rarely results in a good number.

The R-2 value is deceptively low since even a finished brick walls (interior surface & brick) is better than that.
Did the auditor perform a blower door test?
How did they determine and assign values to the existing walls?
Did they remove any outlets to try and measure or look into the wall cavities?
There are a lot of factors that are used even for a simple wall.
Interior film factor, drywall (or plaster) dead air space (if present), any sheathing present, another air gap (usually), then the masonry, and another film factor (gets adjusted for prevailing wind).

Over the past 30 years I have installed A/C and new heating equipment in many older homes and have never missed on unit sizing.
I expect to spend at least 6-8 hours measuring and poking around, than another 8-16 more doing the calculations.
I have my own spreadsheets (and used to do this manually, 1 sheet per room).

The final thing is, what product was the "energy audit" provider selling?

My comments were intended to (hopefully) convey to the original poster that it would be pretty much impossible for him to calculate the overall R-value of his home based on the information given in the original post.

Basically, the problem of the "2x6 stud wall with 6" fiberglass insulation - automatic R-19." In the real world, the R-value for that wall is meaningless - which is something that I suspect few homeowners really understand.

They see R-19 insulation and they have an R-19 wall - what could be simpler?

Now I am even more skeptical because it takes no windows etc into consideration.

Yes, they did a blower door test. We have 5 SQUARE FEET of holes... they couldn't even get a high enough PSI to really measure everything.

If this wasn't all for government grants I wouldn't be so concerned... would work on things as they made sense.... but $$ is tight, a lot of heating spent going out the holes (literally!!) so we are hoping to maximize our grants.

I guess knowing how they calculated is most of the battle in this case! And knowing our 'true' insulation value isn't what is calculated helps us!