Welcome to the Community, @Gavmo9! 
There’s a couple approaches you could try:
1) Add branching logic to the Zap - use a filter or paths to check whether the folder name starts with a certain letter to determine which folder it should be added to. The issue with using filters for this is that you’d need to set up a Zap for every single possible master folder. With paths you can have up to 10 paths that contain a maximum of 3 nested paths (paths within a path) so you could handle it within a single Zap.
You can find out more about using filters and paths to add branching logic to Zaps here:
2) Use Formatter and search actions - you could potentially use a Formatter (Text > Extract Pattern) action to extract the first letter of the project name, and use that in a Find a Folder (Google Drive) action to locate the correct folder. Then in your action that creates the folder for the project, in the Parent Folder field you’d select the ID of the folder that’s found by the previous Find a Folder action. This approach is the most efficient of the two in my view.
You could use the \b(\w) pattern for this. That pattern would get the first letter from a project name. For example, the letter E from the project name of Example Project:
However, I did a quick test and it looks like searching for a folder name that’s only one character long won’t return a match as it needs a longer search value to be supplied. So you’d need to give those folders a longer title. Like “Projects - A”, “Projects - B”, “Projects - C” for example. That way you can select the extracted letter in the folder name you’re searching for like so:
Then you’d use the custom value option in the action that creates the project folder and select the ID of the folder that was found:
Hope that helps to get you pointed in the right direction. If you run into any issues or questions on the above suggestions do let me know! 
