I don't manage action cost % like that because there are values that are managed on a per-round basis, not on a per-action basis (such as ON/DB allocation). Carry-over break the round structure, so if you want to do that, you have to tweak the system so that everything aligns with the new sequence of actions. The per-second action cost in one of the RMC did just that, but I never used it, it was too much bother for my tastes.
In RMSS, the action cost % does not carry over : if you don't have enough action % to complete the action before the end of the round, you can't perform the action, simple as that (the leftover %, if any, is usually spent for movement or situational awareness, but Adrenals can also fill in quite nicely).