The "2-month salary" is such a marketing scheme...If you consider someone who has to pay a mortgage/rent, food, gas, etc, taking a 2 month gross salary for the ring is pretty harsh.
The boyfriend should just get whatever he's comfortable with, it's not really about how much he spends it on.
Back to the original topic...most people I know in the 20's are getting 1 carat.
ITA!
In fact, I read that personal finance experts advise no more than 3 weeks salary:
"Financial experts recommend that couples spend not more than three weeks' salary, or roughly 6 percent of annual income. On the other hand, the jewelry experts recommend Couples with limited finances at the time of their engagement can certainly reset the stone or add extra stones at a later date."
Engagement Ring Advice: free help for choosing your engagement ring
And hey, if 6% seems to low, bump it up a bit...but 3 months salary is a quarter of a year of income! I'd much rather put that money toward a down payment on a house.

Oh, and about the carats...I live and work in SF, and I would say that somewhere between .75-1.5 is the norm.