I forget this all of the time, but the trick is to:
- After clicking "Place Stack Legend", place the cursor on your drawing where you want the upper left corner of the box to start. Then hit Tab on the keyboard and check all of the items that you want, hit "OK" and all of the legends will be filled in. Now you can drop the lower right corner to correctly size the box.
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OK, I didn't read your question thoroughly, so it seems like you are doing what I stated...