Pickup spacing on a Reflex? (Thornbuckers)

I’m toying with the idea of putting a set of Thornbuckers in my Reflex, but before I can order I need to know which spacing to pick… 50mm or 53mm.

I never swapped pickups before, so this would be a first. And I’m unsure exactly what this spacing means and how it should be measured. Distance between the strings?

Also would love input if anyone around here has tried Thornbuckers in a Reflex or other MM guitar!

Cheers,
Kasper