If I remember correctly, up to a tefach exposed (cumulative, not a tefach from the hairline and back) is still in the green zone as far as making a brocha.
In practical application, if you’re a rebbetzin (or just want to look like one) the techil goes just above the eyebrows, (and the ears are covered as well.)
If you wear leggings (no, not tights…leggings) under your skirt, then your bandanna can push the tefach limit to the max!