I really do not see what this CFI was asking you to do as unnecessary or overly complex. It seems he simply was asking you to calculate a W & B from scratch using the POH and you got all bent out of shape about this? Didn't you calculate one of these prior to the flight anyways? Why not take this as a learning lesson, and create your own WB sheet using the example from the AFM or POH? Then make it a habit of taking the 5 minutes to plug in the numbers when renting the 172. I'm assuming that you had to show proficiency with this when you got your PPL, so instead of placing blame on the younger CFI, take some ownership of your skills and realize that you need to sharpen up a bit. If anything this CFI is doing you a favor, he may be preventing you a headache in the event you get a ramp check.