Bob

Don't give up yet. Some of the problems you have with your set up you will have with the lensalign.

A couple of things, the large target on the lens align is 5 3/16" square. The farthest the edge of the ruler would be from your focus point would be 2 5/8". I think your ruler is to far from your target. That will exaggerate whether you are off if you are just a little bit out of square.

Next, forget testing at the longer distances. With the 500mm lens if it is dead on 37.5' you are going to be dead on at 100'.

Now for your results, it took me a while to really figure out how to read the ruler with the lensalign the easiest. Some times the results are obvious. But results will are never consistent. How I do it is find the farthest numbers out that are in readable focus both back and foward. I take at least 20 pics at a particular setting. I use 1 star for front focused, 1 for dead on and 1 for back focus. I also take several pictures in live view focused at 10x to give me a template of what it should look like. I use the template to decide the numbers I will be reading.

After I download the pics I put them up on full screen. Then I literally scroll through the pics judging them and assigning stars, rather quickly as I do not pick over each one. For example at +4 on my 300mm I came up with Front Focus 18 On Focus 17 Back Focus 18. This seemed to be the best average as I went higher to +5 and lower the results would shift accordingly. (keep in mind when I say front focus or back focus, the lens is probably still within tolerances and whatever you are shooting is falling inside your DOF).

Focus is never dead on each time, and if you try and determine the absoulute zero focus point it may drive you crazy. I found it extremely tedious to do it that way.

One other thing you might do, is set up a flash on your ruler remotely. If not a flash run a light out to it and put it where it lights the ruler.

So I see at least two things to change. Get your focus point closer to the ruler. Make sure you have enough light on the ruler. Possibly change to a white ruler if that doesn't do the trick.