In beersmith you will get much better results if you set up an equipment profile for your small batches. If you use the default five or ten gallon cooler/fryer set up then you'll get all kinds of inaccurate results because it still makes all the assumptions for the larger equipment. I'm not sure if the app lets you build equipment profiles or lets you import them from another source. I would image it does.
You'll need to dial in your system based on the results you get from it rather than borrow from somebody else especially for a smaller system. A few ounces of trub one way or the other can make a big difference in some of the profile's assumptions. With BIAB so many factors in your process can give that result, e.g. porosity of the bag, the crush, whether you squeeze the bag, how much you squeeze, etc. I would start off with something in the 15% range for trub and adjust as you see from your results.