It’s not a strict rule it’s just an observation.
Ben Beckman is probably the strongest OG right hand and he’d be below Shanks who’s at 4, so at max Beckman would be 3B range.
Mihawk is 3B range despite being a top tier.
Kuzan is also 3B range almost certainly.
Roger was at 5B so in theory Rayleigh and Gaban could be 4B range, however we have seen the bounties of of subordinates for other 5B characters (Whitebeard and Sakazuki) and even for the top tier subordinates (admirals) they don’t go above 3B.