I think that the eye relief is a constant 20" not 18" But still, I understand your point.
Actually, although it is "constant" at 20 (or 18 or whatever) it is not "precisely" at 20.... As such, you do have some wiggle room.
I understand your point, but even given what I just mentioned, 18 to 20" is comfortable for most shooters. If not, then you can always mount the scope in such a way that you are, in effect, kinda either increasing or decreasing the "constant" distance. Granted you can't mount a scope too far forward or too far back, but you get the idea.
For me, I just love the "constant" eye relief concept and practice. I have a burris 2x-7x and I love it, but then again I hate it because I can really never get it above 4x anyway because the scope"blacks-out." Come to think of it, in your talking about a magical number or magical distance, the problem would still be the same even with a variable scope because, although you may be comfortable at one power setting, you won't be at another one. Do you see what I mean?
Zachary