The 1500 vs 1520 bit is interesting!
Depending on the exact servo, the mechanical center can vary a bit. Next, to get the servo arm at 90 degrees, or as close as possible, the one notch up or down
can make a difference. Finally, due to the way the servo mounts, it can be necessary to use something other than the 90 degree setting for center.
Obviously, next comes the TX and RX. Since Spectrum RX's have a default out of the box fail safe setting, I'd assume that setting must be 1520 for all channels except throttle.
I'd think that the TX, when a model is reset, would revert to factory defaults, plus or minus a calibration factor.
I also use a servo tester with a digital readout for setup. My habit in setting up Flaps does involve initially setting the flaps for half deflection when the servo is "centered".
It can get a bit complicated setting up the flaps up position, in order to avoid stalling the servo against the stops. The TX servo travel settings are useful in preventing such things.