If you stop a set before failure, then to calculate the number of stimulating reps in the set you need to deduct the RIR. So when doing a set with 1RIR, the set contains 4 stimulating reps. When doing a set with 2RIR, the set contains only 3 stimulating reps. Doing sets with 5RIR will produce no muscle growth, and is essentially equivalent to cardio.

If you are dropping a lot of reps from one set to the next, then your rest periods are probably a bit too short. You might still have some CNS fatigue present from the previous set (as well as peripheral fatigue) and this could reduce the effective number of stimulating reps (even if you train to failure).