Are you doing a no sparge mash or are you planning on running a sparge step? BeerSmith treats a BIAB set up strictly to mean no sparge.
If you are planning a sparge step then I would start with the default grain absorption figures until you have the numbers to fine tune the figures to match your actual values.
If you are doing a no sparge mash and using a BIAB mash profile, then set your grain absorption to the default for the standard mash (0.96 fl oz/oz) until you get the actual results. Worst case scenario in both cases is that you will end up with a bit more wort in the kettle and you can make adjustments from there if needed.