One little issue with your method. It assumes, and we all know what that stands for, that both sides will reach their elastic limits at exactly the same time and hence bend by exactly the same amount. But in the real world things conspire to prevent this idyllic set of circumstances.

First aspect would be any differences in the shaping and thickness of the stays on each side due to forming tolerances from the stay tube manufacturing. Second would be the often seen dimpling of the chain stay on the drive side that makes the two sides non symetrical from the get go even assuming the metals are of even elastic limits. And finally because the chain stays are shorter they will see more of the bending loads and reach their limits earlier than the longer seat stays so the seat stays won't be bent at all or not enough and the dropouts end up non parallel.

The Sheldon Brown method has the advantage of being able to move the bending force around so you can bend both the chain and seat stays of each side by equal amounts so that the dropout stays vertical . And by cold setting the one side by half the amount needed and being able to measure that change from the still intact opposite side to ensure accuracy of the job. You can accomplish and check your results with a simple metric ruler and completely ensure that the frame is still aligned to within the thickness of a line on the ruler. And then the opposite side can be bent in turn using the first bent side as a confirmed standard. So you get a slightly more involved but infinetly safer way to do the job that leaves no doubt about the outcome.

The problem with relying on Hooke's and other laws without checks and confirmations is that it leaves too much room for Murphy's law to come into play. And as someone that has bent, cut, carved, machined, soldered and welded lots of metal and wood in my lifetime Murphy is all too quick to enter the mix when it comes to stuff such as this. In my experience any steps that result in Murphy's exclusion are never badly chosen.