From a technical perspective:
The bright sunlight is around 100,000 to 120,000 lux (
Daylight - Wikipedia, the free encyclopedia)
You want to reproduce the plants ideal natural growing environment, so you would aim for this. A 1700 Lumen CFL light will give 164,000 lux at 4 inches. At 5 inches, it would be 105,400 lux.
You can run a more technical simulation with a spreadsheet if you want, to add in the effects of multiple lights. Simplifying things a bit: all you need to know is: lux = lumens / distance in meters squared.
From a more botanical perspective:
You don't really need to know all of that. Just pay attention to your plant.
1. Put your hand in between the plant and the light. Is it hot to you? If so, it's hot to your plant and will physically burn it. The leaves will turn brown & crispy over a day.
2. Are the leaves pointing up (after as little as 6-12 hours) and new growth light green? If so, you are giving them too many lux. For instance, at one inch, a 1700 CFL provides 2,635,000 lux.
You can easily figure out the appropriate distance by placing them 4 inches or so away, and watching your plant. If the leaves point up, move the light away. If not, move it a little closer. Just back off once the leaves start pointing up.
Pointing Up that I'm refering to is not the same as bending toward a light. If plants don't receive a well balanced (from all directions) light source, they'll naturally position themselves to receive the most of it. If they do this, I would consider adding more lights to even things out, but at the very least, rotate the plants every day or so.
