"anti-squat" is more properly described as the projected intersection of the rear wishbone pivot and tire contact patch lines relative to the CG height and it's location within the wheelbase of the vehicle. It is a function of suspension geometry, not shock valving. You will not adjust it without a cutting torch and welding rods.
Shock valving will control the rate of weight transfer to an axle group or wheel position. Adding more "bump" or compression damping at the rear will slow the loading on the tires. Front "rebound" also has an influence on rear weight transfer, just not as much as rear bump.