Next Caster. I did build some DIY slip plates - failure. So back to basics. I marked out the 20deg on the garage floor. I used plastic bags under the wheels. The wheels were easily turned by hand to the 20 deg marks. Measure the camber. Turn the wheel the other direction. Measure the camber. Do the maths. E.g -1.25+1.50=2.75; 2.75x1.5=4.125