that should be put in the engine forum but

no you don't set the needle to obtain a certain desired RPM. you get your desired rpm range from the right prop. you set the needle for proper operation and proper cooling. you set the needle by finding peak rpm then richening it listening or using a tach to set the right rpm drop.

needle valve setting are all about generalization, there is no right answer. it's about experiance, a trained ear, trial and error. but for 4-strokes

after intial break in, running sloppy rich on the ground for first tanks never exceeding 4k rpm. tanks 2 and 3 still on the ground run sloppy rich but leaning out to peak rpm for a few seconds every minute.
After that run the prop your going to use for the plane. lean out to peak rpm then back off (richen the needle for an 800 rpm drop) that is approximate, if its 750-850 your fine, the rpm dynamics is going to change as the motor wears in. at about tank 5-6 you can fine tune the lower end needle for a tick over idle. the low end can be tough to get right since the low and high end needles effect each other, this also is dependant on the fuel tank being set up right, no holes in the line, tank outlet nipple even with the center of the carb on the engine. after about 2 gallons of fuel you can set the high end needle for a 500 rpm drop, but then again your in a very hot place so you might want to stick to an 800 rpm drop.

for the low end needle.
set your high end, peak rpm then richen for an 800 rpm drop
reduce throttle to idle
now pintch the fuel line to the carb and listen to it closley becuase its very suttle
if the engine just dies the low end is lean
if the engine speeds up and dies the low end is rich
if the engine runs for a couple seconds and quits its fine

now advance to full throttle and reset high end.

if you andvance and the engine quits your low end is lean, if it sputters and slowy advances your low end is rich it should advance fairly quickly with no hesitation. turn the low end in 1/4 turn incruments
once you have the low end were you want it richen the low end 1/4 turn, this with prevent flame out on final landing on a low fuel tank.

take it slow and be patient, it will take a good 15-20 min to get the high and low end close, once set leave the low end alone.


as far as the 55ax, one of my favorite engine apart from 4-strokes. the 55ax will do fine but it might be kinda of heavy for it so don't get to slow with it for the first few flights. usually with 2-strokes try to stay with a 2 blade prop. 2 strokes make their power with high rpm and little torque but the 3-blade prop will work. As far a flaperons on the wp p-51. I recommend you test the flaperon affect on the model a couple mistakes high. with that big motor and flap placement you might get into a spin, it can be fun but unwelcome when unexpected.