I mostly make curry from scratch. However some recipes (kedgeree)call for using curry powder. I can never get the mix right.
Is there a standard spice mix substitution?
There isn't a standard substitution for curry powder; all the blends are subtly different, and may or may not include any of a dozen or so spices.
The standard grocery store curry powders all have turmeric, coriander, and cumin in large amounts, and a smaller amount of cayenne or red pepper. Beyond that, they may include varying amounts of cardamom, mustard seed, fenugreek, asafoetida, caraway, cinnamon, nutmeg, garlic powder, ginger, black pepper, or clove. How almost all of them manage to taste like pallid imitations of real Indian curry blends is anyone's guess.
I know recipes are frowned on here, but in this case I think it may elucidate things. Alton Brown provides a recipe which will probably get you a good ballpark result:
Either store together and grind before use (whole seeds store longer), or grind it up into a powder and store that way.
From that base recipe you'll have to tinker with all of the additional spices to get your own substitution.
The thing to do is to visit an Indo-asian grocer's shop and ask for Garam Masala, which means mixed spices. Be careful not to get the mixture that is used in tea.
The general mixture usually includes powdered turmeric(4), coriander(4), cumin(2), cardamom(1), chili(1) and cloves(1/2), and quite a bit of salt (often as much as 50% by weight). English curry powders also contain powdered ginger, usually.
Fenugreek (ask the man in the shop for methi) has a very fugitive flavour. If you are using it (usually as the dried leaves), add at the very last moment before serving the food - the taste disappears in a couple of minutes if you cook with it.
Methi gets included in mixtures to make them smell interesting, but by the time the dish is served the savour is long gone.