I would like to ask my former PI for a medical school recommendation letter. Is it better to ask in person, randomly dropping by their office, or to send an email scheduling a meeting regarding a letter? For context: I used to work in this PI's lab but transitioned to another lab after my mentor graduated. The PI does not typically interact with undergrads as he is very busy and prominent in his field. He likely recognizes my face, having seen me present at an undergrad conference. The bulk of the letter would likely be written by my former mentor, who has already agreed to contribute to my letter.