tmux has some automatic pane layouts: https://superuser.com/a/456799
Is there a way I can modify these layouts? I really like layout number 4, which has a left pane with an 80 pixel width. But I'd like to modify it to be 85 pixels.
You can modify them slightly, for example you can set main-pane-width to 85 instead of 80.
Or you can create custom layouts (create the layout then use tmux lsw to get the layout string, you can then apply it with select-layout). But the number of panes must be the same for this.