Unfortunately... without having experience with the particular kit... we can't answer about exact control throw amounts and mix combinations such as this. I have never had any Chipmunk...

airbrake function is typically a very large deflection setup... adding massive amounts of drag (and hopefully not destabilizing the model...) The small deflections you note are nowhere near an airbrake function. (though they may be useful for getting a slower landig approach...)

Some models pitch up with flaps... some pitch down (more often down) Pre-mixing down with flaps may be quite a serious error.

Remove the elegvator mix from the flaps... slow the model down and lower the flaps. See which way your model pitches. Yuo will also see wghat happens with the airplanes stall speed and control responses. If the model is HIGH, you have TIME to pull the flaps uyp, add power and recver from whatever unexpected result the flaps may give.

Always change just one thing at a time... and test at considerable altitude. when experimenting with control function mixing. When experiment with a mix... I like to do it on one of the switchable program mixes.. so I can flip the switch and kill the mix if it does something I don't like and didn't expect.