On a pure Interest rate perspective, the correct hedge is currently 69 bunds for 100 Tnotes.
I took into account the current OAS duration on BBG (8.17 for Bund and 7.6 for the Tnote) and a EURUSD of 1.1965
However, to trade the spread I would take into account the historical volatility difference. The TY is much more volatile, so I would naturally use a ratio higher than 69/100
But it depends on your trade horizon.