First of all, ring envy is selfish bullshit. I don't care what her friend's fiances have wasted on a ring, and neither should she. Spend as much as you feel you want, but don't do it because of what her friends will say.
Second, leave your girl if she judges you based on the ring you get her, because she shouldn't care if she actually loves you. I'm serious. Commitment is not measured in dollars spent on jewelry. If she knows you're going to propose to her, set some ground rules and discuss the useful things that you can do with all that extra money that would otherwise be tied up in some jewelry.
Third, almost all of the stuff related to diamond/ring selection (2 months salary gross, 3 months net, whatever marketing tells you) has been created and promoted by the diamond/jewelry industry to make you buy more expensive stuff. No one actually follows that (I think a month is much more common), and it's a stupid rule based on assumptions that have nothing to do about your financial situation and everything to do about making you spend a lot on a ring. If you have 3 months salary worth of disposable income to spend on a ring, you might want to do something more useful with it than spending it on a ring (my practical opinion).
If you rolled up to a car dealer and they told you that 6 months salary is how much you should spend on a new car, would you make a buying decision based on that, and would you even take them seriously?
