There is no exact formula but just some quick thoughts you might find useful.

I would consider sharing a list of a questions with the expert ahead of the meeting (like an agenda) so they know and can think ahead of time of what their responses might be. Also, point out things like wanting to be "a good steward of your time" so they realize you are being conscious of their time commitment to the meeting. Lastly, always leave yourself time at the end for scheduling a next step/follow-up with a specific ask (e.g. could we touch base again via a cal in 2 weeks etc.)