Following the guide to measure correct height in a must as mentioned by Jet. See MSD's instructions here
http://www.msdignition.com/pdf/gear_...d_frm22770.pdf
Many surfaces often altered during a rebuild affect this distance and even factory +/- to0lorances when stacked up present a problem if not corrected. Big cams or higher V spring presures also cause this to be an important spec. have fun, it is really pretty easy to do so not a big deal.