Error typing in pixels in the rulers

I was trying to create a three split screen using the rulers as a guide. The project is 1080x1920. 
The first guide was supposed to be placed at 640 pixels. When I right click and type the number in , it places the guide line aroung 1100 pixels. I haven't been able to fix this because you cannot resize the rulers. Also is there a way to make the number smaller between 0 and 200 instead of 500 on the vertical axis. I've checked all the preference and there is nothing there that lets you modify the ruler.I have also tried using the percentage and setting it to 33% but it does the same thing, it places it around 66%.