Ok you'll need to have your note definitions created first for the composite expression to work. I'd start off by creating a note definition "wonshowdownwhenbetturn", make an action in the Turn section for Bet - had to be recipient. Also make sure "Do not take notes for missed opportunities" is selected. Then in Pre Flop / General tab make sure "player had to show down" is checked. Finally in the General tab uncheck "Lost Hand". Save it.
Next rename this definition to "showdownwhenbetturn" and change the description or grouping prefix so it is different (otherwise the notes will merge and it won't work properly). Then in the general tab make sure "Lost Hand" IS checked along with Won Hand. Choose File / Save New.
Now you have both note definitions and you need to create a composite definition. Your expression should be like this: (NC.wonshowdownwhenbetturn * 100) / NC.showdownwhenbetturn
For the Sample Expression you could use: NC.showdownwhenbetturn.instances
Once you make your first one it gets easier to understand!