If you have a value for the various grades for a particular coin, plot those values on a graph, the mark your coin on that graph, between the grades.
In reality, you only need to know the value for the grades immediately above and below your own, and your coin should sit between those grades.
After a while you get good at it. I do it by interpolation between the grades mentally, and how close to the known points your coin is. Knowing that the value graphs are exponential in nature helps.