I would say 200+ to start with, but as long as you're seeing new shell growth, you have enough light. What that actual number is depends on the individual clam. You can't provide too much light, as all clams that we keep are found in very shallow waters and if they're found is slightly deeper water, it's very clear water.
I don't know about the best but mine is under 300 par and growing pretty fast.
EDIT: I would lean toward more is better. I could never keep clams until I added more lights and invested in a par meter. Judging par by eye just doesn't cut it for me. I always think it's higher than what it actually is.