It took me a bit to get my equipment setup. the best advise I have is find something similar, if you use coolers, dupe the cooler, setup.
For the specifics like Deadspace, you need to know how much dead space your mash tun has. Basically how much liquid is it going to hold w/out giving up. This is not how much your grain absorbs, but rahter how much liquid stays if you try to drain it completely.
Boil off, again you'll need to figure out how much you boil off in a set period of time, 20g is going to boil less off than a smaller size pot.
Loss to trub and and chiller is how much liquid stays in your boil kettle post boil.
Unfortunately unless you know your system really well, it's trial and error. It took me 3-5 batches with the same setup before I had all my numbers dialed in. I made a change (adjusting boil off), then used the volumes it recommend, and repeated the adjustments.
If you change stuff, like going from a mesh braid, to a false bottom, you will need to readjust.
One thing that that changed significantly for me, and made my strike water temp measurement was ensuring to take tun, and grain temp measurements, then determine appropriate strike water temp.
HTH,
AO