I started to reply, but as my response was more of a 'this is what I do in the real world situation' and less addressing your questions directly, I ended up deleting it incase I came across arrogant or dismissive.
...however, since the 'get an app' post has been made, I think I'm safe
Mathematics, geometry and physics are all great fun, for a while I used to calculate my bellows extension by way of formula, because it made me feel like a big man. It didn't take long for that to get tired and now I just keep 30cm (because that's all the bellows I have) of a sewing tape measure in my kit. I have marked on it where all the f-stops fall (in inches, 5.6", 8" etc), I also have marked where my four lenses focal lengths are. I set up, I focus, I measure film plane to lens board, if it's past the focal length marking for the lens I'm using, it's quick and easy to make a visual check of how far over and adjust exposure accordingly.
...that's made me a bit lazy, I couldn't even tell you the formula now!