OK, I just installed the backrest on my RKC.
Here is how I did it.
Deciphered the instructions that covered 127 different variations.
Took everything off.
Guessed at the proper mounting hole (2nd from front).
Put everything back together.
Sat on it and said "Oops, that's too far forward!"
Took it all apart and moved it to the most aft position.
Put it all together.
Sat on it and said "Oops, that's too far back!"
Gave it up for the day and said "I'll move it later!"
You'll find that the backrest arms slant backwards into the mounting bracket. The slit in the seat can accomodate a pretty wide range of bracket locations. Don't forget about the tilt adjustment with the allen screw. Use it for fine tuning.
BTW, I'm 5'-11" and ride sort of upright.
Good luck!