Yes, good catch, I really didn't answer his question! The current (rolling average) margin cannot have degraded from the original (rolling 's average) margin by more that the original 30 min power margin.
(Original RAM) - (Todays RAM) < 30 min margin
One more thought. We do this low power check to predict the behavior at high power, without exercising the engine to high power every day. Rules 2 and 3 are potective of the margin at high power without making you check the engine there. Note that in the maintenance manual, after maintenance, you re-do the 30 min check, and start a new rolling average. In other words, you assure power with an actual chack at the required condition, then start tracking it at lower power to maintain confidence in that original high power check.