Rear suspension changes will minimally affect this number, although they can change the frame geometry substantially. Be aware of this effect. Actually, measuring to a single point is a compromise. A better number would be the angle of some reference line through the frame with respect to a vertical or horizontal base line. But for fork geometry, this single measured line is a good working number to gauge front end changes.
For some, this information is well known. For me, it has been a slow process of learning, mostly by trial and error. I never connected all the dots until I read the ThumperTalk thread. BTW, I may not have discovered all there is to know in this area. Actually, you can bet on it. :-)
Discussion is based on stock CRF230F forks and triple clamps; CR80/85R and CRF150R forks and triple clamps (modified to fit the CRF230F); no special frame mods; no aftermarket triple clamps.
Fork Modification - Modifying stock forks