I always use the 1/150 and 1/300 rules and think about uniform air flow from the soffits up to the ridge. I don't like gable vents, pan vents, or power fans as these can be counter-productive and may, in some circumstances, pull in warm air at the ridge, for example. I know Certainteed requires gable vents, pan vents, or power fans be removed, if ridge/soffit venting is added on a reroof (taken directly from their Master Shingle Applicator book). The other top manufacturers (Tamko, Owens Corning, GAF) highly recommend the same. I think IRC recommends soffits but doesn't require it. I would press the builder for soffit vents, and if you need to, use the shingle manufacturer's instructions and warranty paperwork to back you up.