Currently I'm using
tmux set-window-option main-pane-height 30
along with
tmux select-layout main-horizontal
to create this:

This works well for my main monitor but not as well on smaller screens; it would be great if I could use a percentage so that the non-main panes take up 1/3 or 33% height of the entire terminal or something.
Thank you for the great work. tmux is amazing.