Absolutely right - the magic number is 5.
We can't clinch tomorrow - even if we win and Montreal loses, that puts us at 99/42 ROW, and MTL would have 83/36 ROW with 8 games to go. If Montreal wins all other games in regulation, they get 99 points and beat us on ROW. All the other teams that are higher than Montreal can do the same or better.
We can clinch on Saturday - if Montreal loses their two games, they have 83/36 ROW, with 7 games to go, and a max score of 97/43. We already have 97/42, so just a single point in that scenario and we're in.
More relevant is the BOS/TOR clinching home ice scenario. That number is 13 - computed by assuming Toronto wins out and gets 109/52, and then noting that the Bruins with 97 points need 12 points to tie 109 (and hence possibly lose to ROW), and 13 points to win. So, we need at least 7 games to be played by Boston and Toronto, and all seven going the right way...which means the earliest we can possibly know about home ice is 3/27.