LifeHacks

For me, there's two different factors for this, the "you" viewpoint and the "market" viewpoint.

For the "you" viewpoint, you need three numbers: What income is the bare minimum I could live off, how much of an increase would I need from my current salary to move, and what is my ideal salary.

The final one leads to the "market" viewpoint. How much is my employment worth in the job market? To answer this question you need to pull together lots of different bits of information, such as: What's the average income in my area? What's the average income for my job title? What skills do I have to differentiate myself? What is my performance like in comparison to my colleagues? How do I demonstrate my value? What skills did I have to get my current salary, and how have I improved since then to justify more? How important is my role (or how important is it perceived to be) in generating revenue?

Once you have a view of what you need, and what you deserve, then you know what kind of offers are reasonable and what are not.