First measure the rim in mm (enter this figure in the first box- hypotenuse), then hold the spirit level touching the bottom of the rim and measure the distance between the vertical spirit level and the top edge of the rim in mm. Adjust the number in the adjacent box until the opposite side equals your second figure, the number in the angle box is your camber (see image for my example). this gives a close enough result considering you can only adjust in 0.5 degree intervals.
http://www.easycalculation.com/area/triangle-angles.php
Turns out I'm running 2.5 degrees! will wind it back to 1.5 and see what I get.
Alex
ps obviously this only works on level ground.