Hi Arnulf,
the standard removal strategy is managing the available quantities on top-HU Level (see storage type customizing: "Level of Available Qty."), but not on sub-HU Level.
If you Need to have it on sub-HU level during pick-WT-creation you can either enter it manually (as you're already figured out) or you have to use the BAdI in the stock removal strategy (RMS_DETERMINE / RMS_QUANTITY).
Best regards,
Matthias