hey choader,,,yes, you are definitely on the right track there. there is another thing you need to consider: how much of your lamp's light actually reaches your garden. you will be surprised how many of those precious lumens are wasted!
for example, let's assume you have a white-painted reflector - then about half of the lamp's light is reflected, so out of that reflected half you really only get about 60% - the rest is lost in the reflector!
now, to make things even worse, a lot of the lamp's lumens never even get to you plants. they're lost off to the sides, and again, wasted.
i did some tests a while back you might be interested in. There's a chart
here .
plus there's another thread you might be interested in where we're discussing a lot of this
here .