[asuffield]

The rate required to stop experienced developers from leaving. That's quite easy to measure. Enough of the people will leave will tell you what offer they got, and you adjust up until the rate of turnover is acceptably low.

[imaginenore]

From my experience, there's always a company willing to pay you more. If they see some company paying you X, they think your skills are worth at least X.

So your explanation doesn't really answer the question.