Quote from Martinghoul:
One thing to be aware of is that the ZN CTD is, normally, a 7y maturity bond and ZB CTD is, normally, a 25y bond. Thus, using ZT-ZN-ZB, you'll be trading a 2s7s25s fly. Using the newer ultra-long contract will get you closer, i.e. allow you to construct a 2s7s30s fly.
This is probably a silly question, but does the FH function on bloomberg account for this when calculating the hedge ratios? I'm assuming yes...