Using medium chilli powder instead of kashmiri chillis, what ratio should be used

Chili powder is typically a blend of ground chilies and other spices (and maybe even herbs).

I'd go with a blend of:

• paprika (smoked if you can find it)
• cayenne
• cumin
• oregano
• garlic powder

You can also try mixing in onion powder, other chilie powders, and black pepper. Some pre-mixed blends also include salt. If you have access to other dried chilie powders, you might consider using ancho or new mexico chilies for a 'dark' chili powder, as opposed to the more red powder you'll get from the cayenne. (but cayenne's more available)

Search online for recipes -- you'll find lots of variation out there.

From experience I'd say those 6 months don't stack. You could have 6 months in the jar, or 6 months in the freezer - very soon after defrosting the flavour would die out. If you have to freeze I would recommend using it straight from the freezer, if you can avoid it clumping.

The overall potency is degrading from the moment it's picked! But realistically from the moment of grinding, as the oils in the ingredient are slowly seeping and evaporating out of the grain.

Why not make smaller batches more regularly? That's what I do for garam masala, mixing a new batch every 3 months or so.

I would always recommend oven drying spices / peppers very low for a long time - say 2 - 3 hours at 100C (212F). Wet ingredients could be dried in the same way, else added at the time you are going to cook with them.