jraculya,If you can track down a PAR meter, it's much better than dividing by 70 and crossing your fingers.
(Sorry Hoppy, no disrespect.) Just my opinion that the "right" answer is always preferable to an approximate answer if one is serious and success is the driving factor.
I never cross my fingers!
Seriously: I have found it extremely difficult to get a PAR reading that I feel is right on. Today we have LED lights, which use 660nm red LEDs for some of the light, and near UV LEDs for some of the light. But, the Apogee Quantum PAR meter only captures a fraction of those parts of the spectrum, so it is very likely to give too low readings for that type of light. Even for conventional fluorescent lighting it is critical that we keep our hands and arms out of the tank when taking a reading, or the reading will be off due to either the shade from our arms or reflected light from our skin. I was surprised at how easy it was to see that effect. Also, fluorescent bulbs don't produce a constant intensity from the time they are turned on. The PAR readings will get higher for a few minutes, then drop some over the next few minutes, before settling down to a near constant reading. Older bulbs will give a lower reading than newer bulbs.
Just because a PAR meter says we have 27 PAR doesn't mean we have that. It is only accurate to about +/-5%, so 27 means we have from 26.5 to 27.5 PAR, due to the number of digits we get, and from 25 to 29, with the inaccuracy considered, even if there are no other errors. So, it best to round that reading off to 25-30. This begins to make me wonder if my "divide by 70" estimate is about as good as it is going to get.
When you are really serious about light, and want to hold the light intensity to within a narrow band for each plant, then a good PAR meter is the only way you can get it. But, what about the difference in PAR at the base of the plant and the top of the plant? Those who want to know as accurately as possible what light their plants get should still use a good PAR meter, and they should factor in the variation in light over the whole plant. Most of us, who just want a beautiful planted tank, and the confidence that they have a light level that is consistent with the fertilizing, CO2 injection, and plant species they have, can in most cases do that with a cheap lux meter.