To be honest, I would not use the Hallertauer in an IPA. Save them, they are really hard to find right now. They are a really great hop for lightly hopped German beer, a Kolsch for example. You would complete loose the flavor of the Hallertauer if you used them Cascade. If I were you, I would find a nice high alpha to bitter and use you Cascae for flavor and aroma in the IPA.