Particle location on blocks

I'm trying to make a block emit particles on it's surface (like when waxing a copper block) but the dx, dy and dz values are screwing me over. If I set them to 1, particles are emitted all around the block but in like a 3x3 area. If I set them to 0.5 the particles are in the block but only on 1 corner. And if I set them to 0 they're in 1 pixel in a corner.  Does anyone know how to fix?