Haven't used Nebula. Anyway jerky scrolling does not mean wrong refresh rate but emulator not synchronizing to refresh rate. In other words, you can have silky smooth scrolling over a totally odd refresh. The actual reason to create exact refresh rates is to avoid modifying the emulation speed. In short, you need to enable "v-sync", "vertical synchronization", "synchronize to screen refresh", or a similarly named option in your emulator.