If you just want the best light penetration into the canopy.....
From a point source, the power density of the emitted light decays as 1/(d^2). If the distance from the source to the canopy is "d", then at twice that distance "2d" you have 1/4 of the power density.
Assuming that the power density would be the same at the surface of the canopy in either configuration....
If in the low power configuration the source is 12" above the canopy, then at 12" below the canopy the power density will be 1/4.
If in high power configuration the source is 24" above the canopy, then at 24" below the canopy the power density will be 1/4.