If you shrink a PDF instead
Shrinking changes everything on the page, including the grid spacing, the text and the binding margin. The percentage to use is the size you want divided by the size you have, times 100. When the two pages are different shapes, the width and the height give different percentages.
Check both edges
Compare both dimensions in the size guide. If you scale a PDF from another source, check that nothing is clipped and measure the result before trimming.