Dynamic Button is one of my most favorite Sketch plugins. Here is how to use it.
Open Sketch and select the Text tool (T) and write something on your canvas.
Select the element you wrote and press cmd+J. This turns your text into a flex button group.
Open the flex button folder and double click the layer that says 0:0:0:0. This works like padding in CSS, top:right:bottom:left. You can put whatever numbers you want, but for the demo lets use 20:30:20:30. Press return when done.
Now with the text layer selected, press cmd+J. Your button will now get the proper padding based off the numbers you input on the layer. Again, make sure the text layer is selected, not the group.
You can then choose the BG layer and make it look however you like.
The benefit of this tool is when you have to use multiple buttons, with varying text, in a single project. Now you can easily copy a button, change the text, then press cmd+J and your new button gets consistent padding.