For SPs, I have used this formula. For ones with DU of 30 or greater, I add the DU and ST and make that the max, and subtract 5 for the target. For SPs with DU of 25-29, I subtract 10. For lower DU, I subtract 15. This assumes a five-man rotation.
So if you have a starter with 32 DU and 85 ST, I would set the guy at 110/115.
A starter with 25 DU and 85 ST, I would set at 100/110
A starter with 22 DU and 85 ST, I would set at 90/105.
I adjust occassionally, if I see someone is bouncing back slower or faster than expected, and if there is an upcoming off-day.