I put up a very simple guide to FOC for Marcus on another
AIUK thread see
http://homepage.ntlworld.com/joetapley/foc2.htm
As to it's importance: See the following quote from George Techtmikov. People are prepared to pay high prices for tungsten points to get around 1% higher FOC for the same mass arrow.
"The X10 point you saw is a prototype, and is 97% metallic tungsten, not tungsten carbide. It is made by a U.S. defense contractor for Easton under exclusive contract and the design intent is to provide greater FOC with the same point mass. The material is more than 2.2X more dense than normal point materials. A side benefit is that it is also more resistant to damage in the target bosses typically found in Europe."