Well, with each toilet you put a vertical edge up against the back of the toilet (to simulate a wall), then measure from the bolt holes back to that "wall". You could do this with a vanity cabinet in the store, a door, a nearby wall or post, or you could bring along your 4' level and do it that way.

Some toilets will give you extra room to the wall even when sized properly, others less so. (Eg. if one buys a Am. Std. Cadet 3 with standard 12" rough, it's sits quite far off the wall anyway.

Bring my own wall, of course! I was all stumped over the math, and you bring me the common sense of experience. Thank you SO much!

If I allow for the overhang of the tank lid my L-square should be perfect for the job. Seriously, you made my day!

Homebound: you wrote:
(Eg. if one buys a Am. Std. Cadet 3 with standard 12" rough, it's sits quite far off the wall anyway."

Does this mean you could actually put it on an 11" rough-in, or a 10"?

Thanks, Sharon - you made my day, too.

Sue,
Yup, it might fit an 11" rough, but who has that?

The extra space behind the toilet might seem odd at first, but not much you can do after it's installed so might as well get used to it. (And that's when you talk up the positives to a client of having it that way: a) easier to paint wall behind toilet, B) more dramatic effect, C) allows one to build a really skinny shelf right behind it (that you always wanted), D) one of the best flushers money can buy, E) cleans easier, ......

The standard rough-in is 12".

In addition to those mentioned by homebound, is condensation and the restriction of natural air circulation when the tank is close to the wall, which can promote mold growth.

Turns out the toilet was so far from the wall because it was made to fit a 10" rough in. The rough in is the normal 12". One of the few normal things in the place.

Thanks everybody!