It's hard to talk numbers when we haven't seen the maps.
Presumably, the people in the Brotherhood know where their base is and how to get to it better than we do.
Consulting with Jim, will we be able to resupply at the BoS base or do we need to make sure we have enough to make the trek both ways? How long will it take us to get there and what kind of route? Can we walk the whole way, or will we need a boat? Are we going to need to pay for a boat for which we need to bring sufficient money?