Lightroom Book module, Blurb templates: padding to actual image size

I want to place a 5cm x 5cm image (or any other size) in a template and have it print at that size so the caption can say 'actual size'. How do I calculate the amount of padding needed?

I can do this with the full page photo template as the dimensions of the page are given by Blurb in points and can be padded accurately in points. But Blurb doesn't have the dimensions of other Lightroom templates' borders. Those have been created by Adobe. Can Adobe give the size of its templates' borders in points so the calculations for padding can be made accurately for any template? Quite a large chart, no doubt, but necessary for sizing images to actual size when printed.