A lot. Especially if the cam is a very large one with low static compression. I normally shoot from the hip with a good guess in those instances. The "normal" 4 degrees advanced is usually not enough. I wouldn't be afraid to go down around 100 ICL. Maybe even 98. I don't look at it as cheating the cam. It's more cheating the low compression. It can really help cylinder pressure and bottom end torque.