It depends....Watch some replays and identify what seems to be the issues. The adjust the following accordingly.
Run blocking - How frequent a blocker can engage a run block and how far of a push up field they get. If the CPU is manhandling your line, lower this.
Block shedding - How frequent a defender can escape a block. You can also tell this is too high if DL are tackling you much more than LB's. If the CPU seems to be holding their blocks, lower this.
Run reaction time - How quickly defenders pursue the ball carrier. If your plays are constantly getting blown up in the backfield, lower this.
Do the same for your running game.