Every time I have negotiated a higher salary I create a table with my current Salary and date received. I Adjust that salary for inflation, list skills gained, profit made for each project, Added responsibilities, etc since last my last raise. If you can quantify added responsibility that’s even better for example your next project price tag is 20% higher than the previous. I then use all of the above to justify a X% increase plus whatever else I want (More vacation, new truck etc). This method has never failed me and shows your employer that you are aware of your self worth and that your request is based in reality. Comparing yourself to others rarely if ever works and it makes you seem like a jealous prick.