I had the same problem with scrolling...
You can make a custom action and just add the Action "Modify MIDI CC/mouswheel: -10%" a couple of times or "Modify MIDI CC/mousewheel: 0.5x" before the actual zooming action and bind that to the mousewheel.
Problem solved (hopefully)
EDIT: don't forget to check "consolidate undo-points" in the Action editor before saving your custom action.