I tried to go about the way you mentioned, but I don't think it was quite suited for my purpose. The purpose of the rule is to send a notification through Home Assistant to my phone, and I didn't want a delay that I believe your method would have introduced. What I came up with was creating a global variable and a helper rule, that sets the global variable to true when the 300 seconds is exceeded of power being above 3.7.
Then there is a Variable Value check in the Triggers of the Notification rule:
The Reaction resets the Global rule back to false, after the cycle is completed.
It may not be the most elegant method, but it reduces the false positives on my phone when the cycle selector is bumped. For the same reason, I have the reactor uptime checked, because I would get tons of notifications on my phone when I did an update to MSR, or when my server was rebooted. I know I could completely encapsulate the logic within MSR, but the overly simplistic sensor in HAAS serves it's own purpose in HAAS for my dashboard, and HAAS can't read rule states from MSR, so for my purpose, I already have the simple sensor in place, so why reinvent the wheel?