My coach uses a formula to calculate the amount of work done as a ratio to one rep max. This allows us to do a different number of sets and reps each workout, resulting in the same, more or less, amount of work done each workout. You figure this all out based on what percentage of 1 rep max the working weight is as compared to sets and reps. Here is how my coach told me to calculate it.
Take the weight you are using and figure out what percentage of 1 rep max it is. For example for bench press my 1 rep max is 360 and for this example my working weight is 315. That is .875 of my one rep max which is 12% less than 100% effort. Then take the number of reps and sets and multiply those together. If I do five sets of three, multiplying those together I get 15. To determine the work done as a percentage of 1 rep max, divide the sets times reps by the percentage less than 100% effort: 15/12=1.25.
The goal is to do enough sets and reps to get close to a 1.6 work done as a percentage of 1 rep max. My coach said 1.6 is the ideal number and should be the goal you try to reach every workout. In this case we fell short by .4 so we will add sets of 225 x 10 at the end. Alternately you could add working sets of 315 x 3 until you got to 1.6. The possibilities are endless.
By adding three sets of 225 x 10 at the end you calculate the work done as follows: take 3 sets of 10 (which equals 30) and divide it by the percentage by which 225 falls short of my one rep max of 360 (225/360= 62.5% which is 40% less than 1 rep max) 30/40=.75. Adding 1.25 to .75 I get a work ratio of 2, which is well above 1.6. So ideally we should do less work to get to 1.6. If we do one set of 15 that will be 15/40 = .375. Adding 1.25 to .375 you get 1.625 which is pretty close to 1.6. and shows it was a good workout.
By adding another set of 315 x 3 you will have 3/12=.25 and adding .25 to 1.25 you get 1.5 which is pretty close to 1.6 and in this case would be acceptable because you won't always get to a perfect 1.6 on these and having done this workout this morning I can tell you another set of 315 x 3 would have just about killed me so it would have been perfect.
And if you understand this, Mugzy will give you a genius banner and a free month of TID subscription . . .
But I promise you, this formula works and it keeps you doing the right amount of work each workout, even when the weight, sets and reps changes from workout to workout.