Depends on where you want the pinion to be.

If you want the pinion to stay in the same place relative to the car, cut 2" off each side. If you want to move the pinion 1", cut 3" from one side and 1" from the other. If you want to move the pinion 2", cut all 4" from one side.

The best way is to mock it up under the car, measure tape where you THINK you want to cut it, then measure twice (or three or four times) and cut once.

You don't want to be saying "I cut it twice, and it's still too short."