I need a second opinion on my math. The goal is to find the correct length for my new custom Ford 9" rear axle on my 54 F100. Here's what I've measured:
Current rear axle from drum to drum (wheel mounting face) is 61,22".
Wheel mounting face (drum) to bed side is 4,33".
Wheel mounting face (drum) to inside of fender is 6,3".

Now, I want to mount a 12x20" wheel on the rear. Lets say I go with an offset of +35mm which I think is pretty average on a wheel size like this. I also want to mount new fiberglass fenders that are 3" wider than the original ones to be able to fit the wheels with 295-315 tire width.

What is the new length of my rear axle?

I think like this:
Place the axle flange (I'm not mixing the brake disc thickness into account at this time) in the center of the fender plus an additional 35mm (1,38") to compensate for the offset on the wheel.
So the math goes like this: To place the axle in the center of the fender I need to add a length of 2,49". That's because 4,33"+6,3"+3" (3" is the new fenders) divided by two is 6,82". 6,82"-4,33" is 2,49". In addition I need to compensate for the wheel offset so I need to add 1,38" which is a total of 3,87". Does this seem correct?
This means I need to increase my axle width from 61,22" to 68,96" which I think sounds very much...