I used to get two cubes out of my 50L pot doing BIAB.
Trick is to aim for two 17L (ie the 15L cubes) and go for 1.060, then you dilute with 10L or so in the fermenter to 1.048
I used to sparge in a 20L pot.
So do you mash, calculate it so that you fill the pot/keggle, right to the top. Grab 3L of strike water out with a jug so that you can back fill to the top. Mash away.
Pull the bag, dump it into the 20L pot, and fill to the top with sparge water, agitate and mix for about 10 minutes, then pull the bag, add the rest back to your boil pot and start the boil.
Any you can't fit in, pour in as you go.
You can use my CE Biab Calculator to work out how many L and how much grain to use based on your evaporation rate and sparge size etc.
Actually, if you know your evaporation rate, maximum L you can mash, and max L you can boil, let me know, and I'll set up the CE Calc to work out your brew day.
View attachment CE_BIABcalc_2012_04_02.xlsm.zip