There is a website from Macslab on "optimum tilt" that explains it. You need your Latitude.
http://www.solarpaneltilt.com/If you also move a portable or use your twirler, it has a different routine from just aiming it South all day.
With a twirler you want higher tilt earlier and later than at mid-day, but with a fixed tilt, you want it to be lower then than the correct angle for mid-day, to capture some shoulder hours better. Macslab has it all worked out what is the optimum compromise for that by season.
The correct tilt angle for high noon changes with Declination at about 8 degrees a month, so if you stay in the one place very long, you might want to change the tilt angle or pick an angle for the middle of the time you will be there and not for the first day you get there.