Larry,
The shop heads on double-flush rivets will never quite fill the dimple...definitely not like a nice factory head on an AN426 will. There will always be a "halo" of space around 'em. That's normal.
Since you're building an -8 you won't encounter any riveted trailing edges like that. Other models like the -9 and -10, and the -7's "new" rudder use riveted trailing edges.
For the sake of argument, if you were building a -9, I wouldn't "under dimple" and try to get the shop head to fill the dimple better. It'll never look like a factory head. Close, but never exactly.
On the flip side, you might think to use a longer rivet -- i.e. on the practice project try a -3.5 instead of a -3. But even that doesn't fill the dimple smoothly.
Regardless, we're talking about tiny fractions of an inch here. There will come a day when you call Van's for advice on something, and you'll get the infamous response: "You're building a farm tractor, not a Swiss watch."
