The problem with that equation is that it only works for a certain volume for a certain amount of time. With a consistent heat input in a given kettle, water DOES NOT merely evaporate for a certain percentage per hour, but rather, a certain volume per hour.
For example, let's say you start with the same 10 gallons as in your example, but (for the sake of convenience) boil it down to 8 gallons in an hour. Figuring out the percentage boiled off for 2 gallons in that hour will give you a 20% per hour boil off rate. But it doesn't work like that... if you started off with 8 gallons (or simply decided to boil for another hour), you'd still boil off roughly 2 gallons, only this time it calculates to 25% since you're down to roughly 6 gallons. Start with 6 gallons, and you'll calculate a 33% boil-off rate. Then starting with the 4 gallons you have left, a 50% boil-off rate, before boiling off the last 2 gallons in about an hour for a 100% boil off rate!
I'm aware that some software calculates your boil parameters using a % per hour boil-off rate. And while it's not exactly ideal, it's still usable, as long as you understand that it doesn't really work like that. If you figure out your boil-off rate in absolute terms (eg 2 gallons/hour), then a little bit of simple math can give you a "theoretical" % per hour that can be used in such software to give you the same total volume boiled off (ie final volume) for whatever amount of time you're actually going to boil for. But if either the duration of the boil or the pre-boil volume happens to change for a given batch, then said theoretical % will obviously need to be recalculated.
