Undulating Periodization Loading Question

I am trying to figure out how to load with an undulating periodization program. When figuring potential load I THINK (I really am not sure), that I should consider total exercise volume primarily. For example, if on week 1 I do 3 x 10 on flat bench press with 100 pounds for the load, then my set volume would be 1000 and the total exercise volume would be 3000. Next week, if I am set to do 5 x 5, then I THINK i should take the total exercise volume from last week, 3000, and divide by 5 to get a set volume of 600 and then divide by 5 again to get a rep volume of 120 pounds. Using this method, I would achieve the same exercise volume week to week. As I said, i’m not sure AT ALL that this is correct. This is my first time using undulating periodization and I have only been lifting a little over a year, so I’m really just trying to figure this stuff out. If anyone can give me some help with this question, it would be much appreciated.
Thanks, Thad.