officially, in RMC, if you wear multiple armors, you use the highest AT, and the maneuver and other mods from both armors are combined.
blag flag -- I am not sure where you got that formula. Perhaps it was from one of the old Rolemaster Companions. If that is so, then it can, at best, be considered a possible option, not an official rule or official option (this being partially due to the current status of those products).
Also, there is a problem with that formula, in that if gives huges DB bonuses with combinations that are at the opposite ends of the spectrum. For example wearing robes (AT2) under Plate (AT 20), would, by that formula, give a much higher bonus to DB than wearing Chain (AT 16) under plate.
AT 20 - AT 2 = 18 - 2 = 16 x 10 = +160 to DB ?!?!?!?!?
vs
AT 20 - AT 16 = 4 - 2 = 2 x 10 = +20 to DB