Appropriate padding percentage between buttons

I'm laying out 5 buttons programmatically on the screen.

So for example, the screen is 1000px wide. I'm dividing 1000 by 5 and get 200px for a single button. Since this is for mobile devices, it can be of any size and I'm having a hard time determine how much padding should be used here from those 200px.

Note: I'm not considering tablets and landscape.