HeliTester,
I have to admit I haven't thought this much about it in about 31 years (the procedure was developed about 1980).
I believe your last explanation is correct, I don't have a copy of the CSB but your explanation is consistent.
Certainly, the intent of the 3rd rule is to be sure the engine has not degraded enough so that 30 min and 2 1/2 min power cannot be obtained. That is why we get suspicious if the engine's average daily margin slides down by the amount of the margin shown at 30 min power, we suspect the 30 min margin will fail.
The confusion as to what is the "baseline" is perhaps a little academic, because an engine seldom slides down in power if it is kept clean and not eroded, so the difference between the RAM based on the first 5 points and the RAM based on the first 20 points should be very small.
This is a good thread to help me clear the cob webs, thanks!