I think I may have a working solution after playing with it a bit. Here’s the steps, in case anyone else comes looking for a similar solution.
Conversions table, with fields BuyUnit, ConvertRate, UseUnit and the primary key formula {BuyUnit} & " to " & {UseUnit}
Example records: foot to inch, foot, 12, inch
Purchases table has UnitConvert field as a link to the Conversions table. Makes it easy to simply start typing “foot” and see a list of possible conversions. The lookup fields UseUnit and ConvertRate are based on this link. Then I also have formula fields to convert from BuyQty to UseQty, based on ConvertRate.
I hope this helps anyone else! :slightly_smiling_face: