IS Mounts were different front and rear, so bolting the same IStoPost mount on the front resulted in a 20mm bigger rotor compared to the rear.
Post mount frames/forks ‘should’ be the same, as a post mount 160 is a post mount 160 irrespective of if its front or rear. Of course frame/fork designers complicated this by then designing dedicated 140mm, 180 and 203mm post mounts.
So..
To fit a bigger rotor you need to identify the frame mount and buy the appropriate +xx adaptor (no front/rear specific versions exist?), of course some frame designers will not consider you might want to use adaptors or only consider one brand, so you might find some wont fit very well or at all.
it’s luck of the draw and depends on the brake and frame combo?
yup!