how much depends first on your plane and then you....
first how much will you plane take. the flap mixing shouldn't cancel out elevator or plane reaction to elevator. find out how much controlable elevator throw you can get into your plane with out the plane acting crazy and then set that at the high rate point. then set a lower rate for normal type flying. now go back to high rate and start adding the flap function a little at a time untill you see what you want. if you want the plane to snap loop mix it till it snaps. ( not all plans will snap loop) when you find your desired high rate mix go back and do the same for the low rate. then set expodential if you like.
for me it takes a while to find the right rate and mix for my funfly planes. I fly mostly Menaces and each one reacts different to mixing. up to lately ( at slow speeds) i always felt that with the mixing on the plane tends NOT to balloon as much because an elevator command with the mix on is actually distributed through the whole plane rather than just a shift in angle of attack. i flet that if I apply up elevator and the flaps drop besides creating more lift, they also creat more drag there for hindering ballooning to an extent. but on my last menace things seemed to be the opposite. I believe it has alot to do with the cg location though.
I usually fly on high aileron and low elevator with the mix on.
i keep the high elevator cranked real high and only use it for a flat spin or snap loop or an event. it's great for events because i can do a loop right off the runway and the plane just about stays stationary in the air. keeps my landing time down.
that's the way I do it. i'm sure everyone has their own way of dialing in a funfly.
nuturalize your elevator and flaps. fly the plane level see what it does. you may have to work your flaps for level flight then roll it over and see how far off of level flight you are. you may have to work your cg some to get it to fly true nutural..
what plane are you working with anyway...???
you may not be able to set the plane up for level flight both inverted and upright.