Specify pane percentage in tmuxinator project

How can I specify a pane percentage in tmuxinator ?

Eg:

 project_name: ad_dev
 project_root: ~/Programming/WWW/Rails/projects/ApparelDreamDev
 rvm: ruby-1.9.2-p290@apparel_dev
 pre: SQL
 tabs:
   - editor:
       layout: main-vertical
       panes:
         - vim 75%  
         - #empty, will just run plain bash
         - top

eg: the vim pane would take 75% of the screen... is there a way to specify this ? or where in the documentation should I look ? Can't seem to find it anywhere.

The layout should be specified in the layout: line. But you are not limited to the five preset layouts (such as main-vertical). From the man page:

In addition, select-layout may be used to apply a previously used layout - 
the list-windows command displays the layout of each window in a form 
suitable for use with select-layout.  For example:

       $ tmux list-windows
       0: ksh [159x48]
           layout: bb62,159x48,0,0{79x48,0,0,79x48,80,0}
       $ tmux select-layout bb62,159x48,0,0{79x48,0,0,79x48,80,0}

 tmux automatically adjusts the size of the layout for the current window
 size.  Note that a layout cannot be applied to a window with more panes
 than that from which the layout was originally defined.

First set up your layout just how you like it - you can adjust widths with resize-pane until it is just right for you. Then run tmux list-windows. And then you should be able to use the layout: line from the output unaltered in tmuxinator.conf

So based on the output from your gist:

0: tmux [208x73] [layout b147,208x73,0,0[208x62,0,0,208x10,0,63{104x10,0,63,103x10,105,63}]] (active)

The relevant section of the tmuxinator conf file should be:

  - editor:
       layout: b147,208x73,0,0[208x62,0,0,208x10,0,63{104x10,0,63,103x10,105,63}]
       panes:
         - vim
         - #empty, will just run plain bash
         - top