Many of your questions about supplemental lighting can be answered at a University of Illinois Web site.
Research has shown that cows will produce 6 percent to 10 percent more milk if exposed to 16 to 18 hours of continuous light per day, as opposed to 13.5 hours of light or less.
The University of Illinois Web site summarizes the research in this area, including the effect of supplemental light on milk yield and feed intake. It also includes calculations and conversion formulas that will help you set up your own supplemental lighting program.
You may access the Web site at: http://il-traill.outreach.uiuc.edu/photoperiod