I usually try to do 1/4" for AD3-, gives a bit of safety margin over the 3/16" minimum. But as long as it's not less than 3/16" (when drilled), you'll be fine.
Note: On my RV-4 project the flanges were not bent at the correct angle. A lot easier to fix that before riveting (or dimpling, if you take that route). If you can test fit the flaps before riveting skins to rear spar, I'd recommend that.
Finn