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