How to prevent losing solvent into the vacuum pump in rotovap solvent collection?
Honestly, I never bothered to actually measure the volume of the condensed solvent in order to compare it with the original volume.

The concept described in the question, that is using a cooling trap with dry ice + acetone, is good practise and typically good enough to retain the evaporated solvent and protect the membrane pump. In rare cases, I have used a second, flat Dewar filled with dry ice + acetone on a "Laborboy" to cool the condensate-collecting flask.