Tony_F18s diagram correctly shows the rake measurement/setting, but issues can occur when trying to make sure each side is the same. You can get the spreader tube lengths equal but unless the fitting to the mast was spot on it can leave the set-up unbalanced. I have seen them as much as 8mm out on the mast which can make a big difference at the wire end. Set your spreaders as if each side was identical, then do a check measurement from the wire to the luff groove on each side. Compensate as necessary.
I wonder how many people have diligently checked their tube lengths without checking/noticing the mounts were not quite the same?

Cheshirecatman