Strictly speaking, you are correct.It's not that simple. One will need to take the following parameters into this equation: the angle change on your rear swing arm due to different preload
Basically my swing arm bolt went higher from stock, resulting in reduced rake and decrease in the wheelbase. I can't count for an additional rear wheel radius as the pivot point is a swing arm bolt now, not the rear axle.
It can be done, no questions about it. And it's not as hard as with shock linkage bikes where you need to figure out virtual pivot point
But it can be spooked into being simpler.
Visualize the frame in space, oriented as it would be with everything stock.
Load the formula on that basis.
Next, take a step back and re-visualize the frame in space, as reoriented in space by all the changes at the back, be it tire height and/or Rider Sag shock length, all of which do nothing more than alter the height of the frame along a datum perpendicular to the centreline of the rear axle.
Take that difference, double it, and add it to the factory tire diameter.
The results will be close enough for what we are trying to figure out.