Another thought, are you taking these plants directly to flower? HPS should really only be used for flowering, you want MH, or "cool white" CFL's or fluorescent for seedling/veg stages. The spots, to me, look like they might be burns from getting droplets of water on the leaves which magnified the light enough to burn the leaves. Keep a close eye on it, if the spots spread, get bigger, or change color, its most likely a nute issue, but 1/4 strength nutes should be fine at this stage. Keep us posted!