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:    2 tablespoons whole cumin seeds, toasted 2 tablespoons whole cardamom seeds, toasted 2 tablespoons whole coriander seeds, toasted 1/4 cup ground turmeric 1 tablespoon dry mustard 1 teaspoon cayenne   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. 