OK, answering my own questions here! I think I have figured this out and would appreciate your input.
First, the CBS water calculator seems to have an error. The Loss to Boil field C12 is fixed at 3.
I believe that this value should be calculated from your boil off rate (per hour) and the length of the boil. C12 should probably be this equation: =C3/60*C7
The CBS water calculator has a field for Grain Absorption Rate, C7. The default value is 0.1, which I believe must be in gallons per pound. You can make Beersmith's match this value if you look in the Advanced settings under BIAB Grain Absorb. Beersmith's default value here is 0.586 fl oz/oz. You can make this value match the CBS spreadsheet value of 0.1 gal/lb if you enter 0.8 fl oz/oz.
In Beersmith, the BIAB mash profiles will use all the brewing water at once, so I guess that's the base mash profile to start with. Copy one of the existing BIAB profiles to a new Nano Home version and adjust the step temperature as needed.
Now, build a Beersmith recipe with the CBS equipment profile and the BIAB-based mash profile. Then put the total grain bill, boil time, and final volume into the CBS spreadsheet. You should see pretty good agreement between the CBS spreadsheet total water amount and the Beersmith total water amount.
(Your actual boil-off rate may be different than the values in the CBS spreadsheet depending on altitude etc, but enter your true value both in the Beersmith equipment profile and the spreadsheet and everything should work out.)
My test recipe with 10.97 lb grain and a 5.5 gal final batch size came out to 8.66 gal in the CBS spreadsheet, and 8.64 gal in Beersmith. Now, I feel like I can design a recipe in Beersmith and get water volumes that match the CBS worksheet.
Then again, I don't have my Nano Home yet so maybe all of this will fall apart!
Later I will post a modified version of the CBS spreadsheet with the correction I mentioned, plus some edits for clarity. Looks like I can't attach ODS or Zip files here.