2.5" body drop

i am doing a body drop on a 89 b2200 reg cab and i have heard that you need to do a 2.5" body drop to be flush w/ the frame, i have set my 2x4's under the truck cut and folded over my pinch weld and i am only measuring 2 1/8" to be flush w/ the frame, i have my cab cut now and its sitting on the boards, i am just curious if i did not do something right, or if people are going a little below the frame? please help?