If your water is normal tap water it probably has enough magnesium. Look at the water quality report for your water supply department. It should tell you the amount of calcium and magnesium, in parts per million, in the water. If the amont of magnesium is even 1/4 of what the calcium is, or something around that, you should have enough. All of this assumes the GH of your water is around 5 degrees of GH or more.

High light depends on the type of light you use and whether your tank is small, medium or large. If you use lights which are about 2-4 inches above the water surface, of a quality similar to those supplied by AH Supply, with their light kits, and with reflectors also similar to theirs, high light would be above about 2.5 watts per gallon, for a 20 gallon or bigger tank, up to maybe 100 or so gallons. For still larger tanks the amount can be less, and for smaller tanks it should be more. This is a very crude way to measure the intensity of the lights, but it is adequate.

I'm sure others would have a different set of numbers, but not by a whole lot.