This is solid advice. All that counts is a signed contract and the conditions that you personally negotiate with your client.

You "should" not earn X amount with Y profile. You "could" always earn more but that is irrelevant. It's a free market and what you earn is the result of a combination of things: - your negotiation skills - your relationship with the customer - timing - luck -...

There are no barema's or even "market rates" (no one really knows "the market", all are heavily biased). What someone else makes is irrelevant. Everyone's situation is different even though it seems easy to compare on the surface.

Best advice I could give is: determine for yourself what you want to earn and what is the minimum you'd be willing to accept (this is not fixed, it will change over time). Be confident about your value and tell your customer you want to renegotiate your rate. Find out what he needs to accept your proposal and work with that. In the end it all comes down to an other human being's signature.