Bigger Kettle = Bigger Boiloff

Have been brewing 5 gallon batches with a 10g Boilermaker for about 18 months. Buddy and I are moving up to 10g batches so we picked up a 20g kettle.

Last weekend we did two 5g batches, one using the 20g kettle to boil. Holy crap do you ever boil a lot more off with the bigger surface area!

Has anyone else found this? My normal evap rates in Beersmith are 15%. What should I adjust this to? Or will our larger batches "self-adjust" so we are starting with (say) 15 gallons pre-boil to get 10 in the keg (5g boiled off, lost to trub, shrinkage (no Seinfeld jokes please), etc) instead of 7.5 gallons to get 5 in the keg?

I'm pretty sure I'm overthinking this. Maybe it will just take a bit of experimentation.

As you have found larger surface are means more boil off. You are going to have to do some tests and careful measuring to recalibrate your brewing to make sure you are hitting your volumes as you were with a smaller pot.