I came across another cool tool for anyone contemplating solar energy, be it a PV array, or an air or water solar heating panel.
Rather than involving latitude figures this one makes use of drop down menus where you can choose from countries; USA, Canada, UK and Ireland. Then you work your way through a state/province list and a city/town list. Some states like NM have only one city, others like neighboring Colorado have more. Once the location is selected a list with the optimum angle for each month is presented.
The one strange thing about this calculator is that it uses measurements from the vertical. Most computations about panel angles are measured from the horizontal. ??? Go figure on that. I do find it simpler to work from the vertical, but that may have something to do with the fact that I have a vertical pole and sloping ground.
http://solarelectricityhandbook.com/solar-angle-calculator.html (http://solarelectricityhandbook.com/solar-angle-calculator.html)
Another source for tilt information can be found at
http://www.macslab.com/optsolar.html (http://www.macslab.com/optsolar.html)