also, when measuring for push rods, I'd favor a smaller witness mark that is offset from center as opposed to a larger centered mark. As poppy said the idea is to get a continuous 90 degree relationship, so the rocker is not sliding back and forth on the valve tip creating side loads and wearing out guides. The smaller the witness the truer the relationship, not sure how far off center is acceptable. If you get to far of center something is wrong with the geometry and things need to change.
I purchased some checking springs today with an adjustable pushrod, what tool would you recommend for removing the existing valve springs while the heads are torqued down and changing them with the fore mentioned checking springs?
PS: this is my first time changing heads on a SBF, so im trying to learn everything I can before it takes place