How To Size Widget Elements to Percentage of Screen Rather Than a Fixed Value?

It seems strange to me that you can’t specify a percentage value for the X or Y of a vertical box or size box. If you want your UI to be compatible with all screen resolutions, shouldn’t you be able to specify a percentage and then use scale boxes as containers to have everything scale to fill the defined area?
Maybe you can, but I don’t know how? I tried searching Google for an answer a good dozen times and came up empty.

The reason I ask is that ideally, I’d like to be able to divide the screen into two columns and make one of the columns take up 15% of the screen and the other the remaining 85%. All of the offset or size values are exact values in pixels, however, making this seemingly impossible to achieve.