callback mixes aren't the same as line input mixes

This is a follow-up to issue #66 where we added a third mix height to the 2mL tubes. This works when the mix is called as a mix on a separate line inout, but not when the mix is done as a callback--as a callback, it only does 3 mixes at 2 heights, rather than 4 mixes at 3 heights. I'm not sure where else callback mixes are different than line-input mixes, but we generally want these to be the same.