I'm currently of the opinion:

a. specifying "variable" is always better...
b. until you have a very high fitting height

My thinking is that you don't want TOO long of a corridor.

Now, I doubt if you specified "variable" and the fitting height was 30mm, that you'd get a 25mm corridor...I'm sure there's a cut-off somewhere.

But in (b) above, I usually specify "fixed 18" at 18mm and above.

What do you think of that approach?

Thanks!