This is not quite right. If you know the CUP first multiply the CUP by 1.516, then subtract 17902, which you have right, but to get CUP from PSI you divide PSI by 1.516, then add 17902.CUP to PSI:
PSI to CUP
If you add the 17902 first and then divide by 1.516, you get the wrong CUP.
So, PSI to CUP = PSI/1.516 + 17902