Take your lumens and look up inverse square law; tricky. Basicaly when distance doubles, light twice the distance is spread over 4 times the area(this is why reflectors are so important!), so the loss between 1 and 2 feet is proportionaly the same as the diff between 2 and 4 feet. doesn't fit within the "every 12 inch loss" rule I and others use, but is a property of light and is very often used in my profession. get that light down to the plants!
My head is spinning...must...get more...coffee...