Motor Run Capacitor Optimal Size
They're typically marked +/-5% tolerance, so that should give you some idea of what is expected. Here is a (somewhat) detailed article on motor run capacitors. 

When they are damaged they tend to drop in capacitance. 

From a motor manufacturer, a graph that comes closer to answering your question but does not really tell you how much temperature derating you'll have to do etc. 



For example, if the capacitor is too low and the motor stalls it may overheat because it's not cooling itself, depending on the design.