This link has some tables on light loss for certain reflectors over distance.
http://www.advancedaquarist.com/issu...03/feature.htm
For example, this data shows light loss from 6" to 12" for each reflector tested.
Table 3. Percent of PAR lost from 6” to 12” from the lamp
3x3' area
2x2' Area
1x1' Area
Diamond Light
7%
18%
36%
PFO Perpendicular
15%
30%
52%
PFO Parallel
19%
31%
52%
Spider Light
24%
36%
59%
Synthetic Sun
25%
39%
49%