I thought that was the point of standard RODs in the hold - If one a/c reports leaving a level, at say 500fpm, then the next a/c can be cleared to that level, as they will obviously maintain separation.
As you say, leaving and left have very different meanings. Left is once you have 500fpm, AND 300 feet below the level, whereas leaving just means you have a minimum ROD leaving the level (I believe 500fpm, but I would like to find where that's actually written down.
120.4 - I haven't heard of 650fpm +/- 150 before. Is that written down anywhere? I was taught by our training Captains to use a standard 1000fpm whilst inbound to the hold, and once in the hold. This is low enough to prevent TCAS TAs, is acceptable to passenger comfort, but lhigh enough to ensure prompt vacation of the previous level. It was my IR instructor who told me the definition of 'Leaving' a level, as oppose to 'left'.