I always get myself turned around when trying to calculate percentages. Let me start from the beginning -- Adobe Premiere Pro does not have a letterbox plugin. This is irksome to me.
Vashi Visuals has some .png files to download with all the different aspect ratios at all sorts of resolutions.
At first I thought that there were only 4K images in that download, which is cool but we shoot in 3840, so the image has to be ensmallened slightly. How much? Luckily there are calculators on the web for that. The answer is 96%, which is how much I scaled the image to by eye, but it was nice to check the math.
That said, there are "ultra HD" .png files, I just didn't see them because they're at the end of the list and I can't read.
Remember Jarl's Premiere Presets? That's where they are.