Lemon fics do need some kind of descriptor to tell how lemony they are, but I don't think we should rely on a points system in that regard. That adds an unnecessary level of complication to the mix. First we have to get everyone to agree on where the boundaries of each point are (whether an otherwise long story qualifies as a three if it has only one sex scene that's very explicit for instance), and after that, we have numerous arguments over where each fic falls in the system. That increases the amount of time it takes to get a rec added to the index, and then the arguments will probably crop up again some time in the future when someone joins later and has a dissenting opinion. Furthermore, someone who just joins and wants to find a fic in the index has to look up the scale and ask someone for an explanation rather than picking something up based on the recs.
The reason why the +/- system works so well for us now is that anyone can make their opinion heard, and while we'll probably have a discussion about it, so long as it's a valid opinion, we can just make note of it in the index and leave it at that. We don't have to twist ourselves to find a single rating for any particular fic.
Obviously, this doesn't work for rating lemons, so instead of trying to use a concrete scale for lemon fics, I propose we just add a short explanation at the end of each rec in the index. Just one sentence that briefly describes the sex. That way people don't have to twist themselves into knots coming up with a number and we can be more specific to each fic.