I've been doing a 4 day split for the last while. I usually aim for 3 sets of 8 reps after a quick warm up. How should I be weighting the bar? On bench press, for example, I put a 45 on each side. I can do my first 2 sets of 8 with that weight good but the last set I have trouble on and can't usually get 8 reps. Should I just struggle with this last set or take some weight off so I can get the 8 reps again?