Grain absorbs approximately 0.1 gallons per pound (depending on the moisture of the grain). So, you will need approximate 0.3 more gallons.
If it were me, i'd aim a little on the high side, if my boil volume was too much, id boil a little longer to compensate.
Another option is using some "top-off" water to get up to your volume. When you are using BIAB, i don't think a 1/4 of a gallon will get you any more/less efficiency when you are boiling it off anyways.