Tinseth and boil volume - conceptual help!

I know most people use Beersmith or some other software to develop recipes but I don't, I use an excel spreadsheet because it forces me to learn some things. One of those things are Tinseth's wonderful IBU equations. One of the key components of this calculation is the boil volume in gallons. Two others are boil time and boil gravity (to determine utilization).

All or most of the software I have checked use the batch size for boil volume. So for a 10.5 gallon batch, use 10.5 gallons. But it doesn't account for kettle losses. Let's say I leave .75 gallons in the kettle to keep trub and hops out of my fermenter. Doesn't that really make my end boil volume 11.25 gallons? I assumed yes...that if you end with 11.25 gallons of wort, that's what you should use, not the batch size of 10.5 just because you're throwing away .75G of it. However this results in a slightly lower (8%) IBU calculation compared with most software and even Glen's javascript calculator. It appears all of these use the batch size only.

Any thoughts on this? I kind of expect to hear crickets on this one, I should probably just accept it and move on.

I use the tinseth equation in an excel sheet for calculation, and use the average boil volume. Either way, pre or post boil the numbers are pretty close. It's just an approximation any way.

Probably so, but like I said the difference in my case is 8% because of what I leave in the kettle. If you use Hopville.com the difference is even greater. If you are up around 10% difference that is getting significant.

Anyway I can make good beer either way if I consistently use my own spreadsheet but trying to match what I'm doing with others' recipes, the result can be off.