how do people ratio these up.... do traders normally go long/short of two contracs hedging against the reverse in the ''body'' of the fly??
Usually people weight these contracts so that their position is neutral versus a 1 basis point move in the underlying yield of each cheapest-to-deliver underlying bond. Currently PV01 values are: Bund 860 Bobl 459 Schatz 181 So if you wanted to set up a PV01 (parallel yield shift neutral) position of long Bund short Bobl, for example in 100 lots: 100 x 860 = 86000 8600 / 459 = 187 (approx) i.e. buy 100 Bunds and sell 187 Bobs