In practice, the formula does not come out exact re spring length vs. suspension movement. I put shorter, stiffer Rugh springs in my GTV and had unexpected dimensions at all four corners. Not radically off, but not exactly what you'd expect either. In my experience, setting ride height in a 105/115 car is an inexact science, and make take more than one trial to get it where you want it. As you say, you can get pretty handy with the threaded rod. Or buy/borrow/steal the factory tool, which makes front spring removal a matter of literally minutes, once the car is high enough to clear the tool.