The rule of thumb I read somewhere is if the gallon matches the light output it looks like this:
10 gallon with 10 watts = Low (wattage same as tank size)
10 gallon with 20 watts = Medium (wattage 2x the tank size)
10 gallon with 30 watts = High (wattage 3x the tank size)
But I think not all lights are the same and not sure how LED's are compare.