Thats a really tough one to try and answer. Too many variables involved. What coral species, what are they capable of enduring, how close to the light source and so on and so on. For most corals, I would want them to get at least 200+ par, and thats after the light has traveled through water/depth to strike the coral. Instead of risking the corals and trying to make them get by, I would rather you invest in either a T5 system or put a single MH over the tank.
http://www.chucksaddiction.com/lights.htmlCharles