I just put the axle as far forward as it went so it's essentially using it as a vertical dropout. I've got 35c cx tyres and there's plenty of clearance still. I had no particular rationale for doing so, though!
On my cross check I put the axle in the dropout where it gives the tyre (40c gravel king sk) the largest amount of clearance, that happens to be about half way up the dropout.
Soz - they were tiny ones that don't fit over the surly screws :-<