300 bucks? My GAWD, was it solid gold? I use these. I have a few for popular rear ends. They work well. I think your method would be fine. Just remember, the numbers stamped on the end of the pinion are there only for a guide to get you in the ballpark. The gear pattern is the deciding factor. Pinion Depth Tool
I went to link and watched video but it was worthless. T rr ting to visualize exactly how its used . Same as flat stock I assume but you subtract a certain amount?