I use the zone system - I've not come across the 'beyond the zone system' methodologies. But here's my 2p worth...
Presumably, you set a personal film speed in the same way as you would with the zone system? If so, provided that you use that meter for determining your PFS, then it doesn't matter whether it's calibrated at 12, 15 or any other percentage of grey.
I use a 508 and have never had any problems getting accurate exposures.
As far as Tomm5c's response is concerned, I'm not sure whether it's possible to adjust the meter itself. There is the facility to have two ISO settings. I'll check the manual out (if I can find it that is...) to see if it's possible to do a DIY calibration, but as I said earlier, I don't think it's worth it.
I'd stick with the 508.