The Rest between sets is determined by the amount of time required to regain sufficient strength to successfully achieve the minimum effective Volume. There is not much to it, and you will not see any dramatic results by varying rest periods (within reason, of course) Overall, rest periods should be around 1.5-2 minutes. When moving from one body part to the next try to alternate between opposing or antagonistic body parts – commonly known as antagonistic pairing. E.g. 1 set of chest, rest, 1 set of back, rest, one set of chest, rest, 1 set of back, rest etc. This way your chest, shoulders, and tris can rest while you hit your back and bis. After a while you will find your aerobic capacity going up as this closely resembles sort of a power circuit. You may shorten rest periods on 15s to attain the lactic acid effect - on the order of 30-60 seconds. Increase rest periods as you get into the heavier, neural ranges - on 5s and negs you will most likely find it necessary to rest on the order of 2-5 minutes between sets. Also, shorten rest periods earlier in the workout when you are fresh - then progressively increase rest periods as fatigue accumulates towards the end of the workout.