using medium chilli powder instead of kashmiri chillis, what ratio should be used?
I've been growing a variety of chilis recently and have experimented with drying several varieties. By best, Mk. I eyeball guess is that the dried flesh occupies between 1/2 and 1/4 the volume of the fully hydrated flesh.

So I'd shoot for approximately 1/3 as much (by volume) dried chili as fresh (assuming that you are comparing to finely chopped fresh chili, because the dried stuff does not retain the shape of the original).

Of course for small thin chilis that you can dry whole (like the little Thai reds) the substitution is one dried chili plus some moisture equals 1 fresh chili.