How do I know my rate at slush so I can set my gpumax price properly?

Slush is proportional so your effective PPS rate fluctuates depending on the total number of shares it took for each round. You'd need to take a running average of your payout in terms of the number of shares you submitted for each round vs the total number of shares in each round. Slush does not give you that information so you're pretty much boned on getting actual numbers.

You can try adding up the 'U' numbers for all of your miners and multiplying the total by 1440, let's call this value 'A'. Then, add up your rewards from Slush for one day. Call this 'B'.

Divide A by B, and you've got a (very rough) estimate of your PPS rate. You'd have to do this for a number of consequtive days and average out the PPS rate to get an idea of what you're actually earning since Prop pools are subject to variance.