The margins are percentage, as you said. For the image you posted, you’d have to keep margins at 0.4 or 0.35 - eyeballing it here, you may get away with 0.3
A margin 0.4 for an image 100x100 pixels would mean that when you stretch the image, only the 20 pixels in the middle will be stretched. The 40% from all sides will just stick to the edges.
To get a good result, it’s always better to produce a base image that will never be scaled down, only up - basically make the smallest you will need and 9-slice that. Yours looks spot on.
edit: pixel size is weird because of how lazily I copypasta this 
51 is 25% of 204. Your margins should be 0.25 or more. The text cannot be a part of the image, of course.