just checked your radio manual spitfire66  JRsx600  and it tells nothing about  mixing aux flap channels..only mixing for delta wing etc..

         Unfortunately  I don't think you will be able to have down elevator trim mixed in when flaps are deployed using that radio.....You are going to have to try to hold slight down elevator in manually  when descending.  . ...

         Just a thought   !!    When I  trim my models I trim for level flight  at full throttle .Some people like level flight  trimmed  at 3/4 throttle ,which means as you throttle up you climb.... If you trim your spit as i do pulling back to 3/4 throttle  will make you automatically descend  which will help when you then deploy flaps.......Personally I deploy flaps at around  3/4  to   2 / 3  throttle and on  down wind leg and then down to 1/2 throttle on up wind leg on final  steep descend , once past the thresh hold  throttle goes down as I previously explained..