Couple of things to add: turn off autopause on the Edge, turn on zero averaging (this only affects the numbers displayed on the device and not the data you download to analysis software, but having zeros excluded will massively inflate your power numbers on any normal ride),
Whoa ...! Steep leaning curve ahead!
OK, I've turned off the Autopause and switched on the zero averaging. I'm not sure what the latter does but from what you've said I guess it sort of smoothes out the data to eliminate such as freewheeling periods - ?
turn auto-zero on and always manually zero at the start of a ride.
I can't find these options in the Edge menus (unless you refer to Resetting the data in the Trip Log?)
Does the G3/705 pairing show you the zero offset value when you do a manual zero or does it show the device ID? If it shows the zero offset then this should be 512 +/- 12 (assuming its the same as for the old Pro+ / SL+ models). You can check for drift due to e.g. temperature changes during a ride by looking at this number (more relevant for turbo rides where you don't coast since on road rides if you coast the Edge should reset the zero point if it has changed).
Nope, sorry, I'm completely lost now! Does "offset value" refer to the figure I get when I hit "Restart scan" in the Accessories screen? (I don't have the Edge in front of me as I type, so I might be recalling this wrongly.)
I'm also unfamiliar with "drift" - as a newbie and without any documentation with the Powertap, I'm somewhat in the dark about all this, sorry!