I think looking for a specific formula may only confuse things. The trick with the blues is that it's a fairly simple approach to music with a lot of hype surrounding it.

As an acoustic blues guitar player I don't work from pre-memorized arrangements. Before a song kicks off (and this is the same if I'm playing solo or if I'm playing in a group) the only information I want or need is the key and the time signature. The time signature gives me a hint about the rhythm and the key gives me a general outline of what the chord progression "might" be. For example, if the song is in E I know that I might be working with some combination of E, A and B.

I don't look at songs in terms of twelve or six bars. The song is what it is at the given moment. If I'm in the rhythm all I have to do is feel out the basic flow of the chord progression and go with it.

Once that happens that fancy-freaky chord stuff is just a side effect of whatever is happening in the song.

I'll Take the E chord: 0-2-2-1-0-0 and the E7: 0-2-2-1-3-0 and mess around with the melody line by changing the chord in some way.

In other words, in any E chord I might be playing variations like: 0-2-2-1-0-4, 0-2-2-1-3-4, 0-2-2-1-0-2, 4-2-2-1-0-0 or a bunch of others - but in my head it's still just an E chord. There might be some highfalutin theory to explain why the variation I'm using at a given moment works, but I could care less about that stuff. Knowing theory isn't the same as knowing the craft.

My advice would be to take some basic blues stuff and start playing. Get comfortable with the music. Feel the flow of it and then start messing around to see what you can and can't get away with.

-Patrick
www.howandtao.com