Sublime Text custom pane layouts (commented with wireframe explaining coordinates for further customization). File should be named and placed in the following location to prevent being overwritten with future updates: "/~/Library/Application Support/Sublime Text 3/Packages/User/Main.sublime-menu".

README

/* ==========================================================================
   Example: 4-cell layout (3 col, bottom-left 2/3 row)
   VIEW WIREFRAME: http://go.acc.bz/sublime-layout-3col_1row
   ========================================================================== */
   
{
	"command": "set_layout",
	"caption" : "3 col, bottom-left 2/3 row",
	"mnemonic": "C",
	"args": {
		"cols": [0.0, 0.3, 0.6, 1.0],
		"rows": [0.0, 0.75, 1.0],
		"cells": [
			[0, 0, 1, 1],
			[1, 0, 2, 1],
			[2, 0, 3, 2],
			[0, 1, 2, 2]
		]
	}
}

/**
 * 0-1 values for COL breaks. 0 is left edge, 1 is right edge.
 * [LEFT EDGE, CENTER VERTICAL BREAK(S), RIGHT EDGE]
 */
	"cols": [0.0, 0.3, 0.6, 1.0],

/**
 * 0-1 values for ROW breaks. 0 is top edge, 1 is bottom edge.
 * [TOP EDGE, CENTER HORIZONTAL BREAK(S), BOTTOM EDGE]
 */
	"rows": [0.0, 0.8, 1.0],

/**
 * 0-n values for coordinates of each individual cell (an array of arrays).
 * 	Each array of arrays [x1,y1,x2,y2] defines coordinates for each cell.
 * 	x,y coordinates represent each break's position in relation to the total
 * 	number of break points on that axis.
 * 	The value for "n" can easily be determined as follows:
 * 		- X coord: Number of values in the "cols" array, excluding 0.0
 * 		- Y coord: Number of values in the "rows" array, excluding 0.0
 * 	This example has 3 horizontal break points, and 2 vertical break points.
 */
	"cells": [
		[0, 0, 1, 1], [1, 0, 2, 1], [2, 0, 3, 2],
		[0, 1, 2, 2] //__________]
	]

/*=====================  Wireframe notating coordinates  =====================*/

             0.0                    0.3                   0.6              1.0
              +--------------------->+-------------------->+--------------->+

             0,0                    1,0                   2,0              3,0
        0.0   +----------------------+---------------------+----------------+
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         |    |                      |                     |                |
         v   0,1                    1,1                   2,1               |
        0.8   +----------------------+---------------------+                |
         |    |                                            |                |
         |    |                                            |                |
         |    |                                            |                |
         v   0,2                                          2,2              3,2
        1.0   +--------------------------------------------+----------------+

/*
   End of example 4-cell layout (3 col, bottom-left 2/3 row)
   ========================================================================== */

Main.sublime-menu

[{
	"id": "view",
	"children": [{
		"id": "layout",
		"children": [{
			"command": "set_layout",
			"caption" : "3 col, bottom-right 2/3 row",
			"mnemonic": "C",
			"args": {
				"cols": [0.0, 0.4, 0.74, 1.0],
				"rows": [0.0, 0.75, 1.0],
				"cells": [
					[0, 0, 1, 2],
					[1, 0, 2, 1],
					[2, 0, 3, 1],
					[1, 1, 3, 2]
				]
			}
		},{
			"command": "set_layout",
			"caption" : "3 col, bottom-left 2/3 row",
			"mnemonic": "C",
			"args": {
				"cols": [0.0, 0.3, 0.6, 1.0],
				"rows": [0.0, 0.75, 1.0],
				"cells": [
					[0, 0, 1, 1],
					[1, 0, 2, 1],
					[2, 0, 3, 2],
					[0, 1, 2, 2]
				]
			}
		},{
			"command": "set_layout",
			"caption" : "2 col top, bottom 2/2 row",
			"mnemonic": "C",
			"args": {
				"cols": [0.0, 0.5, 1.0],
				"rows": [0.0, 0.75, 1.0],
				"cells": [
					[0, 0, 1, 1],
					[1, 0, 2, 1],
					[0, 1, 2, 2]
				]
			}
		}]
	}]
}]

Usage:

Place the code in a new file the file named "Main.sublime-menu" located in the "Packages > User" directory (to avoid being overwritten from SublimeText updates).

Custom A:

Top L.	Top R.

Lower Full Width

Custom B:

Upper Full Width

Low L.	Low R.

Reference Links:

set_layout method...the coordinates explained | Sublime Forum
Code author's original explanation | Stack Exchange
Plugin: Origami...pane customization & automation | GitHub
Configuring and Mastering Split Windows | TutsPlus