Bruce, the article I used for my experiments with Whitcombe tiplets gave the following -

Tiplet root - -11*
Tiplet tip - -4*

The most successful set that I made were jigged on the wingtip at -7* > 0*. I tried the reduced toe-out on the basis that the out-flow angle of the tip vortex would be smaller and tighter than with a fullsized a/c (scaled naturally).

What I think most people who try these things lose sight of is that they are NOT just "glued on" at 90* to span line (right angles to the wing).

My earlier F1A;s tried this and it did not work a dang. So I tried setting the wing design to give sufficient lateral wing area with a 12* dihedral. Now why 12*? Because the Whitcombe tips are set at 78* to the spanline, plus 12* = 90*, ie vertical to the horizontal plane. Why do this? So that I didn't have to factor the tiplet area into the total wing area.... "Part of the wing? Nah!!! They are forward fins..."

But one thing that I did prove was that the combination of rake (sweep), camber(from spanline looking from the front), and angle of attack (toe-out) was critical to them working.

BTW the wing airfoil used was ThomannF4.