I do the same as Chris, 3-5 points softer, but have used one that was as much as 15 points softer than the rest also. My rule of thumb has always been that the hardest tire goes on LR, then the RR is a point or 2 softer, then RF is a point softer than RR, and LF is the softest one of them all. So my sets would be something like 60 LR, 58 RR, 57 RF and 54-55 LF just as an example