mmckenna
I ♥ Ø
No I would probably not need that much at all. Maybe 15’ to get out to the front of the 5er and 35’ it I go toward the back by the ladder rack and what ever height I need. Probably only need 50 to 75’. But as slowmover has said maybe a couple of extension coax’s in case something or camp locations change. I like the idea of having a permanent mount as well as a temporary one.
I'd keep the runs to the length you need. Yes, you can always add an extension for those few times you need it.
LMR-400 doesn't roll up very small, so consider your storage limitation. Also, it's heavy and if you are using this on a temporary mast, you'll need to secure it carefully, or it'll risk damaging the antenna connector, connector on the coax, or the mount.
I use some commercial HF radio fly packs at work that get set up in the middle of nowhere. The antennas are a military model and a temporary mast system called a "rolatube". System 75: Single Tube Mast (Compact & Portable) - Rolatube
That setup uses about 40 feet of a high quality RG-58. Higher grade cable isn't necessary and the importance of it being packable/transportable takes priority.