I’m not sure if this is the best way or will get what you want, but maybe you could use a crop filter. That way you can type the numbers in for right, left, top, and bottom, to make sure they match each other if that’s what you want. However, you can’t invert a crop filter, I don’t think, so depending on what type of mask you’re trying to do, that may not work, or you may have to put it on a white or black solid and mess around with the blend mode. I’m not an expert there so just throwing out some possibilities.