There isn’t a fixed design rule for white borders on photo prints. Border width is largely an aesthetic choice and tends to follow changing style preferences rather than strict proportions.

A good approach is to choose a border that looks right for the specific image and for how you plan to present it. You can also look at what photographers in your local scene or communities you follow are doing if you want to match a common style—or deliberately do something different.

If the print will be framed, another common option is to print the image without a built-in border and use a mat board in the frame instead. That often gives you more flexibility and a more traditional presentation.

So: no universal ratio, just visual judgment and presentation preference.