I was talking to Michael (Southern) about this question and without going into alot of detail, Gels and Fusion are callibrated a bit differently. If you look at the two charts side by side, levels 1-6 in Fusion goes like this:
1-black
2-dark brown
3-medium brown
4-light brown
5-lightest brown
6-dark blonde
Gels:
1-black
2-DARKEST brown
3-DARK brown
4-MEDIUM brown
5-LIGHT brown
6-dark blonde
Fusions level 5 is lighter than Gels level 5. According to Southern, gels runs a shade darker than Fusion at the lower to mid levels and then catches back up at the higher levels. Even though Redken is a 10 level system, according to what Southern told me; Redken snuck in a lighter color with Fusion because Americans have lighter hair at levels 5-6 than Europeans. Gels even though it's a 10 level system, tends to run darker and more opaque like a 12 level system but is not as opaque and flat looking as other color lines. Fusion is more translucent in tone and tends to not give the great grey coverage as gels which is also why Fusion came out with the NN series for grey (which I LOVE LOVE LOVE).
Southern I hope I'm explaining this correctly because you filled my head with a bunch of info that is still bursting at the seams here...lol
Ok...so moving on and getting back to the question of how to formulate...so when working with the gels mid-to low levels, formulate 1 level lighter than desired level up to level 7. Then at level 7, formulate at the correct callibration of level per level. Still use 20vol for 2 levels, 30vol for 3 levels and so on. So...when when going 2 levels lighter use 20vol and formulate 1 level lighter on levels 1-6....make sense?
