How to make an absolute path relative to a particular folder?

Yes, you can do that, it’s easy, think of your paths as URIs:

Uri fullPath = new Uri(@"C:\RootFolder\SubFolder\MoreSubFolder\LastFolder\SomeFile.txt", UriKind.Absolute);
Uri relRoot = new Uri(@"C:\RootFolder\SubFolder\", UriKind.Absolute);

string relPath = relRoot.MakeRelativeUri(fullPath).ToString();
// relPath == @"MoreSubFolder\LastFolder\SomeFile.txt"