I leave the Garmin recording for the whole session, if fact I have always used 2 Garmins just in case one file gets corrupted for some reason. I click the lap button each time I pass a particular lamppost - I can do that without moving as the Garmin is at the top of the aerobars where my hands sit.
The advantage of having one file its that you can see the whole file at a glance which can show you the macro trends and just zoom in and increase the resolution to a run of 5 laps and then each individual lap. I’ve never been interested really in finding out absolute CdA but comparing clothing or positions against each other.
This is a file of an ABAB test (the first 3 laps were getting a feel for the conditions)
By keeping the virtual elevation roughly level of the two ‘B’ sets of 5 laps you can see that the ‘A’ sets (1 and 3) slope downward which indicates a lower CdA You can drill down as far as you like by zooming in to get more accurate data but sometimes the macro level can tell you all you need to know. This was a test between a long sleeved triathlon speed top and a short sleeved trisuit.
The actual elevation (the green line) is irrelevant really as its a loop and by definition starts and ends at the same elevation. Checking the constant elevation box and getting a straight line for the elevation sometimes makes things easier to see.
Bear in mind I’m an enthusiast and not an expert who enjoys going fast off not many Watts so check everything I’ve said for yourself!