Originally Posted by ziggy13
...in my previous experiences I lose about 10% an hour, give or take.
Not to be a PITA ziggy13, but I would recommend that you stop thinking of boil off in percentage terms. Think of it in gallons per hour or liters per hour. The percentage idea breaks down if you adjust your volumes. For a 10-gal batch that starts at 12.5-gal like you're doing, 10% is 1.25-gal which would get you in the right volume ballpark for what you're doing.
But what if you're doing a smaller 5-gal batch that starts at 6.5-gal? You won't boil off .65-gal. You'll still boil off at the same rate you would a 10-gal batch. It's not a percentage, it's a specific volume.
I'm not like Dan. I don't get all the sludge and trub into my fermenter. I know it'll settle out, but I don't want hot and cold break in my fermenter. That's why I figure out my trub volume. I don't know what your keggle's diptube looks like, but I would recommend that you keep track over the next couple brews how much is actually left in the keggle after your rack to your fermenter. This will allow you to really dial in your volumes.
I whole-heartedly agree with Dan though about boiling water for an hour see how much boils off. I do full volume tests for my own system because propane is cheap (for me) and I also want to factor in the evaporation that takes place while the water is heating up. Boiling 2 or 3 gallons will give you boil off amount, but boiling 10 gallons will give you boil off amount AND what you can expect to lose in steam and evaporation over the 30-40 minutes that the water is heating up to boil.
Whatever you do though, take Dan's advice and boil some volume of water and do it a few times and take notes. That will give you a good idea of boil off.
Also, I don't know what you're keggle design is, but 1.5gal of trub seems high. Maybe that's what your system leaves in the bottom. Take a measurement after you transfer the wort to the fermenter. That way you will be that much closer to having your number pretty well dialed in.
I've just recently started taking extensive notes every brew session on this stuff. I do it first because I like to. But second, it helps in diagnosing problems and creating, adapting, and adjusting recipes to come out correctly on YOUR system.
Just a few suggestions. FWIW.