While we work on having the Anesthetic sheet auto-save the timing of premedications, you can time them by:
Add all pre-meds and inductions as soon as you are in the Preop window. Then "Start" some pre-meds. And at that point, there should be "Save" button instead of "Start Anesthesia" (because there will be items in Induction fields that are not yet started). By pressing "Save" everything in Preop view will be saved, and you will return to Flowsheet. After that, you can return to Preop view (from Flowsheet) when you are ready to induce, start inductions, and press "Start Anesthesia".