When to raise preflop?

[OneCentRob]

I play mainly 10 handed ring games - all NL from .5/1 to 3/6. I make a steady profit in excess of 5xBB/hr. However, having recently invested in Poker Tracker I find that I am only raising 4% of the time preflop. Most people seem to think that it should be above 7%. Where should I be getting this xtra 3% from? Does anyone know any good threads, links or articles I could read on preflop raising on mendium stakes NL holdem online? (I tend to play multiple tables, and consequently play much tighter than I would on just a single table.)

Any ides/suggestions?

What is the optimum preflop raise % on this type of table for a tight player who play aggressively after the flop?