To do this, you change the angle you are flaking. Instead of pushing down, you want to push into the glass. If you look at your beer bottle bottom from the side, you will see that it is now, more or less, hexagonal. It has two flat faces and steep bevelled edges. It will also have a slight curvature to it, with a concavity on the bottom face and a slightly convex top surface.
Arrowheads are, most often, lens shaped in cross-section.
To achieve this lens shape, you need to get rid of all the concave curvature of the glass. In the process you will also be covering the blank faces with attractive flake scars. Most of the work you need to do is on the bottom, concave side of the glass. It will be very tempting to remove flakes from the upper, convex side because flakes love to travel across convex surfaces. The flakes you remove from the bottom will be very short by comparison, but that's okay.