your question is best answered with a par meter, unfortunately. the decor, plants, depth, etc. of your tank are all going to be factors. heck... even a submersible lux meter would give you some kind of clue... but unfortunately distance, angle and light vs. air/water are far from the whole picture.
other than for engineering sake... spread of light vs distance above the tank i don't think is really too important with a high-light tank... except in the event you want to shade a lower light plant to keep it from getting algae.
in a non-co2 tank though, you typically don't want the full intensity of the light exposed to anything. placing the bulbs too close to the water in this case can certainly lead to a tank that starts with bits of algae on the spray bar and top leafs and then spreads across the tank like wildfire since the algae has a stronghold and the plants can't consume enough co2 to support the growth to out-compete algae.
for non-co2 tanks 6"-12" above the water is a good starting point for your bulbs.
as well... i've seen many people complaining about not getting the light spread they're looking for... mentioning things like cfl's cause unsightly bright spots, etc., etc.. as far as i'm concerned if it's visually displeasing, doesn't reach certain plants... it can be moved.
i have 2 tanks with the same footprint. 36" X 18". both have plants that reach the top of the water. on one i have 2 cfl's in clamp lamps. in the other i have a t5ho setup with "feet" or "legs" that keep the bulbs ~4" from the top. my plants seem to do just fine in both of them. i just keep the lights off centered more towards the front... but almost centered. this gets the light around my driftwood to support plantlets just breaking the surface of my substrate.
as well i think it was a good point about keeping the light off of the glass.
