Wet Macro Diopter; location of focal point

Many add on wet macro diopters utilize a clamp mount on the port end.

How critical is the location of the wet lens focal point with respect to the lens focal point?

And what are the effects if it is 'off position' along the lens axis?

I can only comment on my combination. I have used the Subsee +5 and +10 diopters in two ways, attached to flip adapters and screwed directly into the port. I guess the difference in distance to the front element might be +/- a few mm or so. I can see no difference in either method.

Same - I can't tell the difference between using a flip adapter and direct to the port (Nikon 105mm and Nauticam SMC on full-frame camera).

If you try looking through the diopter lens with your eye and move it closer / further away you'll find it doesn't make much difference until it's quite some distance from your eye. I did have all the optics equations somewhere but a practical demo will have to do for now!