Factor for production dip away from 180 degrees?

Is there a rule of thumb for solar production as the panels are tilted away from the ideal azimuth? Say, for every 10 degrees off of 180, your production drops "x" percent.
I know that there are very specific tools that I can use for very precise array azimuth / tilt for a given latitude, but I figured there must be some quick and easy approximation somewhere for this ...
I know that there are very specific tools that I can use for very precise array azimuth / tilt for a given latitude, but I figured there must be some quick and easy approximation somewhere for this ...
Comments
Your best bet is to try various scenarios in PVwatts etc to see what you get. OTOH if you are interested in the math these sites will give you more bed time reading than you can ever use in one lifetime:
http://solarpaneltilt.com/
http://pveducation.org
http://pvpmc.sandia.gov/
http://zoneblue.org/cms/page.php?view=off-grid-solar
http://solarelectricityhandbook.com/solar-irradiance.html
PVWatts® Calculator
-Bill