Richard -

Well sort of. Your math assumes that the bulb is an open bulb with no reflector - and no surrounding walls (white or otherwise) to reflect the light.

If you have an enclosed canopy, you will only lose light these ways:

Thru the side of your tank (put mirrors on your tank!)

From radiation heating - basicially, the heating of the surrounding items (walls, reflector, water, rock, fish) by ineffecient reflection of the light (that's why the sun makes you warm - skin doesn't reflect well...).