Arrowhead concavity setting

Okay, I'm stumped. I've been trying to figure out the geometry of the arrowhead and what the concavity setting is based on. The length of the arrow is based on the width of the line of the arrowhead, and is applied to the actual line length - not the actual arrowhead length. The width is, again, based on the width of the line. However, I've can't get the concavity. Somehow it's based on the width of the arrowhead, but I can't figure out how or what dimensions are used to calculate it.