Never reforge mastery into something. Mastery is your best stat along with stamina.
Reforge hit and expertise to mastery if the item does not have it, otherwise dodge or parry, depending on what you have less of.
Only reforge avoidance stats to mastery. Exception is if one of your avoidance stats is more than, say, 1% above the other. Then you can reforge selectively to remedy this.
Do not reforge to hit or expertise. Exception: you need to be a designated interrupter (which you won't be).