Formulas present a result and are going to be separate from data entry / data selection fields. It sounds like your problem might be better solved with Form Rules, rather than a formula.
I would set it up so that you would have a multiple choice drop-down for Flexible or Optimized and a formula similar to this:
If ([FIELD 1] = "", "General",
If ([FIELD 2] = "", "Fluid",
If ([FIELD 3] <>"", [Drop-down Field],"")))
Then, I would setup a form rule that worked like this:
When [Field 3] is not blank, Require [Drop-down Field]