Data Validation Custom with Formula

I am working on a vacation planning tool that sets an accrual cap (AA8) from a hidden worksheet table based on years of service (cell F8) entered by the employee. The fields involved in this are:
Projected Yearend Service in cell W8 = F8+1
Annual Vacation Accrual in cell Z8 is =VLOOKUP(W8,Table.VacationAccruals,2)
Vacation Accrual Cap = Y8+10 (because employees can have a maximum of their annual accrual plus 10 days)

Then the employee enters the hours of vacation that they expect to have remaining at the end of the year (cell F16). I need to put a data validation on cell F16 that it is NOT to exceed the Vacation Accrual Cap.

I set up a data validation using Custom Settings and in the Formula field entered <AA8, and it causes any amount that I enter to be an error,whether valid or not. I also tried <Z8+10, same result. It just isn't liking my references to a cell that has a formula in it.

Can someone help?

I thought you could only use Custom when referring to a cell with a formula, but your approach works perfectly.

Thank you very much,
b