Calculating percentage of a value based on another value.

Hi there,

I've searched a few threads on here about calculating percentages, but nothing seems to quite summarize what I'm looking for.

I have several polygons, each containing an attribute of "TotalKm" and "MinorKm"

The "MinorKm" represents roads inside that are of a lower class or quality.

I simply want to calculate the percentage based on the "TotalKm", like this example:

I tried playing around with the ExpressionEvaluator to do this, but I could't quite create the correct expression for this.

Thanks in advance,