The ZB contract specification says the underlying deliverable is: "U.S. Treasury bonds that, if callable, are not callable for at least 15 years from the first day of the delivery month or, if not callable, have a maturity of at least 15 years from the first day of the delivery month." In the current environment of low yields, the cheapest to deliver bond is on the short-maturity side (15 years). To verify, when I plug in the current 30-year yield of 2.68 to the pricing formula in excel "=PRICE(NOW(),NOW()+365.25*15,0.06,2.68,100,2,0)", I get 140.79 for 15 years, and 168.14 for 30 years. 140.79 being much closer to the current market price, I used 15 years as the maturity to calculate the theoretical limit of 190. However, I have no prior experience with ZB and this calc is based on some help I received on an ET thread along with a bunch of stuff I read in the past week. I may not be getting ZB pricing just yet!