First question: What is the cure temp of the ink you are using? What is the ink?
I ask, since many "low cure" inks actual cure temps are somewhere between 250* - 300*. Knowing that inks actual cure temp is good info to have.
As for retention times. A setting of 8 will give you a 1 minute retention time (each number on the M&R dryer equates to feet per minute. So 8 = 8 feet per minute). The longer you keep the garment in the dryer, the more precise you can be with your dryer temps.
Instead of running "hot" and fast, you can run cool and slower.
I am personally a fan of running your dryer temps closer to the actual cure temp of the ink and slowing down the belt. This way you do not spike the temps on the garment itself. This allows you to not worry about bleeding on delicate poly, possible shrinkage of the garment, melting delicate fabrics, etc.
Hope this helps Scobey!