you're doing this at a weight that is 30 to 40 of your one rep max so the easiest way to calculate that is to take the rep the the weight that you can barely get 10 reps with so what is your best 10 rep weight divide that by 0.75 so your best 10 rep weight is about 75 percent of your one rep max and then you multiply the resulting figure by somewhere between point three and point four depending on how strong you are