Dimple, don't countersink
As a side note, I highly recommend dimpling the tank baffle and skin instead of countersinking per the plans. The worst looking rivets on my RV-8A were along the aft baffle with the countersunk holes. On my -14 tanks I dimpled everything. It went together easily, looks beautiful, and both tanks passed the leak test with no issues.
Spring Hill, KS
RV-8A N2D #80583 flying with G3X Touch / GTN650
RV-14A #140017 wings