I do step mash as I know it quite often biab.
I have a false bottom so bag cant be it direct contact with gas heat
I monitor the temp rigorously turn the gas of 1.5 deg from temp wanted as heat rises and continuously stir till rest period temp is reached It seems to work but it is hard work...
I factor in 14% boiloff and do a 90 min vigorous boil that I only have watch closely for the first 5 mins so I can toddle off and do other exciting things like mowing lawn etc .
50 l bock recently required 81 l of water for the 16.5 kg of grain it wont fit in a 100l pot so I mash in 65 l of...
I do biab double batches 45-50 l finished if I do a 90 min boil the required water full volume is any where from 60- 70 l plus the 10-12 kg grain is about all a 100l kettle will handle without risking boil over
I had a 70 l pot that wasn't big enough for doubles ditched it
just when water is water I asked some questions at the place about my water in Yamba and got a very nice response from the lady there water ain't water in this town as depends on where you are how much chlorine you get etc.
actual numbers she provided was great
I can't see why you cant do
i'm about to do a RIS of about 37 l but with a 100l pot biab i'm going to to do two mashes and mix with a 4 hour boil 19.5 kg grain