In theory, you are absolutely correct. In practice, there are a couple of problems. I think there are some variations in the capacities of tanks due to minor variations in the production. Also it is very hard to establish where "full" is, both when you are looking at published useable fuel and when you are measuring your own. The extrapolation is also a little tough to be super accurate. It is not as liner as we would like. But this is not much of a problem at the lower fuel levels, as it takes a few gallons before it begins to show on the stick
But overall, this method will get you awfully close. We all like to know, down to the ounce, exactly how much fuel is there, but in practice, nobody (with good sense) ever flies that close to the limits.
I still think the best way is to fly one tank down to where the engine sputters. (And no, there is no real danger here. It is only a little spooky the first time you feel that loss of power. Remember, the early POHs approved the process) Land and add fuel in one or two gallon increments, marking your stick as you go. I also suspect to be super-duper accurate, you would have to have a separate stick for each side.