The sparge water is calculated as what's needed to meet your preboil volume - specifically:
sparge_vol = boil_vol + grain_absorb - kettle_top_up + mash_tun_deadspace - mash_vol;
where:
boil_vol is the target pre boil volume
grain_absorb is the volume absorbed by the grain during the mash
mash_vol is the total volume of water added during the mash
mash_tun_deadspace is the deadspace value (from the equipment profile)
kettle_top_up is the top up water (from equip profile) added to the kettle pre-boil
--
Brad