Is this for a report or for calculating the actual deduction for the check during payroll processing? The Deduction setup on the employee record can handle the calculations for you without a formula by using a "Calc method" of Gross Pay, enter 15 in the amount column and enter 114.75 in the "Limit" column and select the limit period (Check, Week, Month, etc).

Say the loan is for $500. I want to take no more than $114.75 until the $500 is paid back. How do I handle that part without a formula?

Say the loan is for $500. I want to take no more than $114.75 until the $500 is paid back. How do I handle that part without a formula?

Use the MIN function in the formula, and be sure to use the PR Check record (as that is the record in memory when you're processing payroll), selecting the Gross Pay field from the PR Check Record. This formula should work:

MIN (Gross Pay * 0.15, 114.75)

You can then use the "Limit" field to put the total loan balance, which will not be exceeded.
Familiarize yourself with the "Functions" by searching the Help menu. There are many useful functions for accomplishing results that can't be done with manually created formulas.