Easy question my a$$
...and to add to that, I am NOT an expert, so take my advice as such
I would determine my required water amount based on my water:grist ratio (e.g. 8gal). To that, I would add my MLT deadspace, whatever that may be (may or may not be space under your false bottom). That would become my total mash water (e.g. 8gal + MLT deadspace).
Additionally, with such a large space under your false bottom I would make sure to recirculate prior to closing the lid to make the mash more homogeneous.
That's how _I_ would proceed, FWIW.