I put together an autosplitter to be used in an "All Main Missions" category. It uses RTA, so it includes time spent in the menus. See the readme on GitHub for more details.

Autosplitter link: https://github.com/crashb/neon-white-autosplitter

EDIT: You can now download the autosplitter directly through LiveSplit without having to visit GitHub. Right-click the LiveSplit timer, then click "Edit Splits..." then select "Neon White" in the first dropdown.