I need help with spacers

Ok, so how do you decide on what size spacers to get?

Do you have a calculation for it?

Do you guesstimate it?

Or is it as simple as busting out a ruler and run a straight edge against the fender along the wheel?

I got pretty conservative fitment going on and I want it to sit flush.

Measure with a ruler, just remember that 2-3mm can either be your worst nightmare or wettest dream. And playing with spacers can be addictive=)