Me personally, based on your numbers, I would do 3 sets, 15, 12, 8. Add 5 pounds each set. And next time do 3 sets of 8 with whatever weight you put up on that 3rd set the previous week and stick with either 3 sets of 8 to 10 or 3 sets of 6 to 8. Don't feel stupid using the 2.5 plates either, every bit of weight is progression.