[Bongle]

Major/Minor radii are measured in pixels in the cameras field of view. The camera's field of view is about 44.5 degrees. With that knowledge (and an assumption that each pixel takes up an equal # of degrees), you can compute how many pixels the 38 inch target will take up at whatever distance you're trying to optimize for.

It takes a bit of trig, but it's not tough to do.

See the attached image for a diagram.