30 lumens per square foot. (Brighter over workbenches)
80 lumens per watt for a T8 tube
40 watts per tube for a 48" T8.

I'd use two twin-tube 48" fixtures between the bays, twin tube 48" fixtures every 6 to 8 foot over top and perpendicular to the bench and try a 42W CFL (200 W equivalent) in the existing alcove socket. (I've never gone with minimum lighting)
JMHO

FWIW I just redid the lighting in my 16x30 workshop. I used 10 4ft LED T8 equivalents. This provides a good general lighting level. It's hard to have too much light in a workshop though so if in doubt, add more. You will want spotlights (either portable or permanent) for doing detail work. Also beware of glare and shadows - one of the reasons I redid the lighting is because the original lighting was all in the center of the room... so depending where I was it was easy to shadow what I was doing. I used single bulb fixtures so I could spread the bulbs out all over the ceiling.. with fluorescent fixtures going that route costs a bit but just a consideration.

We bought Cree LED fixtures that look kind of like fluorescents - only better. They were more expensive, but like the brightness and energy efficiency.

Thought I might chime in on this one.
The actual level you would want in a garage for automotive work STARTS at around 70 lumens per sq ft. If you are doing general service work with auxiliary lighting, that should suffice.
I am in auto restoration professionally and when we laid out our new shop addition we spec'd 100 lumen per sq ft and actually wound up going to 125. To give you some reference, 30 is the recommended level for a table or counter in a home. 70 is what you see in most grocery stores, 100 to 150 is what is recommended for "detailed mechanical tasks" by the IES (Illuminating Engineering Society).
The good thing is it can be accomplished relatively cost effectively by maximizing reflectivity. Light colored epoxy floors, light colored walls and ceilings, and even spacing of fixtures all aids in good light distribution. Cooper lighting has an online lighting layout program that works pretty well. GE has one as well. The thing to remember is that the best work environment will be even in brightness and shadow free, with no glare.
In a home garage, you are better off with a higher number of 2 bulb florescent fixtures than a smaller number number of 4 or 6 bulb units.
A tube with a high CRI (color rendering index) and a higher color temperature ( 4100 to 5000 Kelvin) will also give you a higher quality light. Light quality is what makes a space,"pleasant" to work in. I recently did my basement woodshop and have 150-160 lumens per square foot and you do not feel you are in the basement. Well worth the investment.
Don't forget, your supposed to ENJOY working on stuff in the garage!