Weapon mastery

[Anubit]

I am wondering how much difference is there for example betveen 12 hammermastery and 15.Any way of calculating this(posibly in %)?

The Game Mechanics (http://www.guildwarsguru.com/content/game-mechanics-id674.php) guide over at GW Guru has some pretty good info.

It says there's about a 10% increase in base damage on normal attacks if you have level 15 attribute compared to level 12. Having lvl 16 is about a 15% increase. Any skills based on that attribute usually increase a few more points too.