You are correct that you cannot change it due to having the BIAB with full boil checked. The next areas to look at is your equipment profile for losses to trub and chiller, mash tun losses, and boil off rate. You will also want to measure and record your water absorption from grain, as this will impact how much water initially gets fed into the unit.
You commented that you know the PicoBrew calculator is correct, is this from previous brews using your Picobrew? If so, then you should follow that guidance and measure your volumes throughout the process (water in, wort collected (preboil volume), post boil volume and volume to fermenter). From these numbers you can calculate the actual losses and apply them to your equipment profile.