2",2.5",3" z frame 67-72 c10 ?? how much??

6" frame 3" would be 1/2 for the z but ive seen the number 2.5" being thrown around for the z. whats too much and whats just right.. how far in front and behind the cab is the best spot to cut fot the z .. any advice would be appreciated.

how ever much the crossmember hangs below the frame is how much you need to Z it..just do some measuring

2.5 on mine