From the BrewSmith help pages:
"On the last page (step 3) you set your Brewhouse Efficiency. A good starting point is 68-72% if you don?t know what your efficiency for the overall system is."
Thus - Trial and error in more simple terms as per the instructions

Trial and error, boil off rates in a humid climate, different OGs from different people milling, and malt extract not fermenting down below 1.020 are all things I wish somebody told me about when I first started which is why I am posting.
Again, I think the software is generally awesome, and I am not making fun of it. I was a newbie not all that long ago, and some very small pieces of advice from some very nice brewers in the microbreweries and from the homebrew stores has gone a LONG way in improving the small batches that I make.
I just can't wait until this thing called "work" doesn't get in the way, so I can try something else on a Saturday (hopefully next month).