From the measurements I took on my 6.5 Gal Anvil, there are 3.28 liters below the bottom level of the basket. You can use that to configure your recipe for grain volume, boil off, and trub left after the boil, to see how much water is left in contact with the grains for mashing. In my system, I would not do anything under 6 liters at the end of the boil. I did try a 5-liter batch, but ended up with a bit more than that at the end of the boil and a very thick mash (so therefore a low efficiency) even with a full volume mash.