How to batch equal canvas size increase on non-square images

I regularly need to input a 10%-20% increase in canvas width (or height) with images that aren't perfect squares, yet keep a consistent white border. For instance, I want to batch-process 100 images with varied ratios. I need the same pixel dimensions on all sides after specifying a percentage on one dimension. The only way I can do this is by noting down the horizontal dimension in pixels, adding 10% to the canvas size, then subtracting the original horizontal dimensions from the larger new horizontal dimensions. The resulting number I then need to add to the vertical dimensions. Sounds complicated, but actually a simple and necessary action to give an equal white border on many images with different image ratios. Surely there's a way to batch. Any help on this?