Padding or Margin values by %

I’m new to Elements and am trying to recreate one of the visual elements from an older RW site. I have screenshot frames consisting of a screenshot and a bezel image that overlays the screenshot, making the corners of the screenshot with rounded corners.

The screenshot needs to have padding specified in %, not the units provided by the spacing pane.

The override values don’t allow text entry—only dragging to change the numeric value.

Spacing must be provided in % because these screenshot frames can scale.

(Using a round corner style will not work; I’m specifically using a ‘squircle’ curve shape, not an arc.)

Baring making the % changes here, I can’t figure out where to add CSS class that would do the same.

Thanks,

John