well you did not say what wattage it is so I can only give you the formula so you can do the caculation yourself.

From 0-12" the lumens are the same it's from 12" to ? that it starts to go down.

Here is what you are looking for.

Lumens is a mathematical formula that measures the bulbs intensity at 1 foot. The color or type of bulb is not calculated in the formula I=L/D² Lumens received are a measured in watts-per-square foot or foot-candles. Calculating foot-candles or lux is a more accurate way to estimate the amount of light plants receive. This still only measures how much light you plants have available but not how much your plants use.

Light diminishes exponentially fast!

So how close you can place the light is really determined by the temperature at the top of your plants. 12? or less is all the same. So it??s 12? to 48? we have to balance between heat and lumens penetration and the closer to the 12? mark the better without burning the leaves. If there is too much heat coming from your light then add a glass heat shield and heat exhaust fans to either suck the heat our or inline to push the heat out. How long you vent tubs are will determine weather you push or pull the heat out. You are better off if you blow air through your hood. This will allow you to get the light as close as possible but watch the temperature!