When you want to print, go to Image>Size, and make sure "Resample Image" is not
checked. Then type in your width or height. Assuming that "maintain
proportions" IS checked, the other dimension will automatically fill in.
If you tell it you want it 8 inches wide and it sets the height to 10.4, you'll
have to trim .4 off the picture to make it come out exactly 8 by 10, unless
you're willing to distort the picture by stretching it to the proper shape.
Incidentally, you say you set the print size when you start to work on a photo.
If you don't intend to actually print it from Photoshop, that size is totally
irrelevant. It simply does not matter. You can just change it whenever you
want to print. Make sure Resample Image is not checked, then you can tell it
print 8 by 10, then tell it to print 4 by 5. Both copies should come out
exactly the size you specified, but the image itself will not have changed in
any way.