How calculate NPK of organics in a mix?

Hi there. I was just wondering how to calculate the npk of a organic soil mix. I know I can multiply them by their portion and add them together to get the total npk value but what about the slow release Vs fast release of nuts in different organics or minerals. Lets say we have a mix with different meals and some rock phosphate or green sand or something with a slow release rate. How do I factor that in my calculation? For example in farms if they add any manure or meal that might last up to 6 maybe 12 month (depends on the amount of course) but if they add rock phosphate to their farm it lasts up to 10 years till all the K in the rock is gone. Do you get what I mean? I just wana be able to calculate rough estimate of my organic mix NPK. thanks

im stoned and don't want to be rude,..but that's a pretty technical question and many of folks wont read um if their too long. just info