A min level for most plants is about 25-30.
Max levels are about 450-600.

Full sun = 2000.

Also, if you notice the peak in the middle of the graph when measuring the length of the tank under the bulb.

Since the source (the bulb) is a linear tube, how might this graph change if it was a MH HQI bulb?

Also, consider multiple strike angles here.

If you add 2 bulbs, one on the front and the other on the rear of the tank, how might this influence light intensity?
Measure in the middle, the front and the rear at the same depth.
It should be fairly even now.

Rotate/change the orientation of the probe also as different angles(leaves do not always point directly straight up). Then try it with a single bulb and then with 2 bulbs spaced far apart(like at the front/back of the tank example vs one in the middle or both in the middle).

You can predict the light intensity pretty well vs distance.
Now try patterns from the source of light, angels, reflectance etc and what a real plant leaf does through time.

General physical patterns are fine and do help, but biological patterns are far more interesting and relevant.

Regards,
Tom Barr