USBC only gives restrictions on Differential Radius of Gyration which is at 0.060 inches. This is the difference between the low Rg (x-axis) and high Rg (y-axis) of the bowling ball.
The intermediate differential refers to the difference between the high Rg (y-axis) and the PSA (z-axis).
In theory, I believe the PSA can be as high as 0.059 inches assuming it is less than than the low Rg (x-axis). If at any point the PSA becomes smaller than the low Rg then the PSA becomes the low Rg and the low Rg becomes the PSA, if that makes sense.