Your weapon reach is 5' plus weapon length in RMC.
IMO that's an approximation off of "Height plus weapon length"
IMO that's an approximation of "Span plus weapon length" i.e. the distance from striking fist surface to fist surface across your chest.
IMO "Span" is a better measurement of reach than arm length, which likely explains why it's the "reach" measurement used in boxing.
If you only punch with your arms, you want arm length, if you presume you rotate and bend your torso, and move your legs. . .
If you nail one of your feet to the floor, allowing you to lunge and strike, you will discover that. . . .the distance from the "Nailed" foot to the striking surface of your fist is actually within a few percent of your "Span".
Which seems to show that ye olde 19th century boxing statisticians knew what they were doing when they measured reach as "Span". (In checking out the background on this, I discovered that while on average in most people span and height are the same or close to it, with world class boxers their span is almost always larger than their height, indicating being abnormally long armed is apparently an advantage in boxing).
That said, I think Height + Weapon length would be correct, but the RM2/RMC rule is the (man sized) approximation of 5' + weapon length, and I can't locate a specific rule in RMSS/RMFRP.