It is hard to be exactly sure unless you and your system in certain conditions have done it before. I usually get about a 1 - 1.25 gallons an hour for a normal boil. Could be more for a rigorous boil. For what you are asking I would say about 3/4 a gallon? Just a guess though
I also get about 1 - 1.25 gallons of boil loss per hour. You could do an experiment, put 3 gallons on the boil and then measure it after 1 hour and you'll have an answer.
If you've boiled in your pot before changing the volume or time shouldnt change the boil off rate so you should lose half as much as you would expect in a 60 minute boil.
As for the grain loss its ~0.125 gallons/lb (this changes a little depending on crush, whether you squeeze drain etc).
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