For the record, in Pathfinder 1.0.2 (1.2.0? I can never remember the version numbers, although there was definitely a 2 in it) included a mod called Not Enough Resources, which indicates that the best Y-level to mine at for lapis, as well as several other ores, is 55. Even at that Y-level, it's still quite rare- at the moment, I'm quite far into the pack, and I've found two whole veins of the stuff, as well as at least ten (quarter-)stacks of copper ore and two gigantic veins of Manganese.
There's also a smaller peak in lapis ore density centered around y=25 or so; but there's still not quite as much lapis at the level than at 55.
I'm not sure if this is still the case in later versions of the pack, but since I haven't gotten around to updating yet (and, from what I've heard, NER was removed for some reason), it's the best info that I can give. If anyone's up to re-installing NER on the current Pathfinder version and confirming my info (just look up the recipes for lapis ore in NEI), please do.
There's also a smaller peak in lapis ore density centered around y=25 or so; but there's still not quite as much lapis at the level than at 55.
I'm not sure if this is still the case in later versions of the pack, but since I haven't gotten around to updating yet (and, from what I've heard, NER was removed for some reason), it's the best info that I can give. If anyone's up to re-installing NER on the current Pathfinder version and confirming my info (just look up the recipes for lapis ore in NEI), please do.