Eric, can you add some detail to your question? I don't know what you mean by "X" dim. I'm guessing you mean the upper block? Usually called the "spinnaker halyard block" (or "sheave" if integral to the mast).
The spinnaker block is just a bit above the forestay. The class bylaws tell you where it needs to be for class-legal racing. The legal value is no more than 15' 5" above mast datum