I have a recipe for making a madras, and it suggests substituting chilli powder for the chilli's, but doesn't suggest how.
Wondering of anyone can give me a pointer as to what ratio to use?
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.
This recipe almost certainly means ground chilies aka chile pepper powder, NOT the spice mixture that is also called chili powder in American English.
Cayenne Pepper powder, or a "Deggi Mirch"/"Lal Mirch"/"Kashmiri Mirch" powder from an indian grocer, will do here. They are in the same order of magnitude when it comes to heat. Western style cayenne pepper powder could be lacking in flavor, while kashmiri mirch powder could color the curry too strongly - a mixture would be wise.
Use whatever amount of the powder you are comfortable eating in a curry of that type.