I usually follow the Annie Duke 30BB rule and I buy in for 180 at a 3/6 table. But usually have another 200 or so in reserve for the next day.
At 6/12 I buy in for 340 just because it fills up one rack nicely.
You could also take the approach of buying in for alot more than 25x or 30x to portray a LAG image or the intimidation factor.
Here is a link to the Annie Duke article on buy-ins.
http://www.ultimatebet.com/learn-pok...ml?articleID=6