I always cool it first before transfer with an immersion chiller. Don't know why I haven't done it this way before, just never thought of it I guess. DUH. I haven't done a partial boil in quite awhile and most of the time I can hit the 5 gallon mark pretty good with the full boils, but once in awhile have to add a little to top off to 5. Last time I did a partial I did not have a valve on my kettle which is why I probably topped off in the carboy and have been doing it that way ever since. One of those aha moments for me.