I boil off a gallon per hour and always do 90minute boils.
I do 10 gallon batches, and loose a gallon between break, trub, etc, so I aim for 11 gallons at end of boil.
So I start with 12.5 gallons at start of boil. I average .1 gallons per pound absorption, so it's pretty simple math to figure the amount of "strike" water:
12.5 + .1 * pounds of grain
When I do a partial mash, I generally just aim for 3.25 quarts per pound as long as it will fit into my pot. (I wrote a little script that figures it out for me.) I do this a lot as I bought a 15 gallon kettle rather than a 20, like I should have. But a few pounds of DME is no big deal... So, make sure your pot can handle the volume if you feel dirty for using a little extract