when you do the exercise 2x5-8, what he means (i think) is you start with a weight you can press for 6 reps without assistance, but only press it for 5 reps. The thing with strength training is that going to failure usually isnt the best idea. so lets say thats 115 x 5 on the flat bench. Next time you do flat benching aim for 115x7 or even 8 reps. When you can manage to get 8 reps for your sets with the weight then you add 10lbs, this should put the reps back down to 5. Dips are a great assistance exercise because they work the front delts chest and tri's just like benching does. If for tri's your doing CGBP, dumbell extensions, skullcrushers, and pushdowns and sometimes dips all 3 sets of each thats probably a bit much maybe you should cut back a bit, unless your taking any assistance?