Help with changing canvas from 2:3 to 4:5 ratio

For some reason, I'm finding this more difficult than it probably is...

DSLR's shoot 2:3 which obviously ends up wider than the normal point-n-shoot 4:5 (or whatever makes the 8x10). The issue is I'm shooting some real estate photos and the MLS site shows the photos at that 4:5 ratio so the images are stretched out height-wise. Instead of cropping off the sides, I'd rather add a white top/bottom border so the pic doesn't distort.

I'm sure I can make an action to add a certain % of canvas with white, but I'm not sure what that percentage is. Anyone know?

Figured this out, just FYI

2x3 to fit a 4x5.3 (not truly 4x5) comes out to added to the canvas size about 112.5% to the height.

I don't know about that 112.5% figure, doubling the size of the framing? But if you figured it out and it works for you, good for you!

To do this in PS is straightforward and I've just done it to get the numbers.

In PS go to Image >> Canvas Size
Make a note of the width of the image - my case 982.13mm
Multiply this figure by 4/5 (or 0.8) - 982.13*.8 = 785.704mm
Enter this figure in the height box.

Make sure that the Anchor is in the center and that Canvas extension colour is set to white.

There you go.

Alternately if you have not yet completed the shoot my advice would be to frame the images in camera so that you can crop in post to get the required 5:4 aspect ratio with the correct framing.

I am not keen on adding an artificial white edge top and bottom, much better to shoot with the final aspect ratio in mind and simply crop to whatever pixel dimensions are needed.