Determined depth foor countersink by utilizing scrap aluminum purchased at Lowe's. The countersink depth is correct when the test piece is flush with a dimpled skin hole. Do not drill such that a rivet is flush in the spar but that the spar is flush with a dimpled skin hole! The hole will be much deeper than a flush rivet hole as the dimpled metal on the skin needs to sit in the countersunk hole. Countersinked first spar piece.
