Use text field to define formula

 If I understand what you are attempting to do, you were suggesting that users can freeform in a formula into a text box and that would then be used in a formula as how to do a numeric calculation.

Interesting approach but I don't think QuickBase can handle that.

I suggest instead that you provide the user with the multiple-choice box as to which method they would like to use for their calculation. Then you will have a formula which does the calculation according to how that multiple-choice field is set.p

 Then I would provide a way for the users to request new methods of calculating the goal perhaps just sending you an email. And you would add these different choices to the formula. But there's not going to be a practical way for users to write their own formula in the same field.