Rick:
I have a bunch of pictures from three different digital cameras. I open each one in Paint.net and then hit save as. The suggested size is frequently different. Ones from my camera are often 62% but sometimes 60, 85, etc. Quite often if I save at the suggested 62% I get a jpg about 1/10th the size of the original. However when I look at them both and zoom in on them both they are identical quality. That seems magic to me.
Regardless of the suggested percentage if I decrease that percent the picture will immediately begin degrading. If I increase that percentage the picture will not improve. I recently went through about 60 jpgs and it works this way for all of them except sometimes it suggests a higher percentage and, of course, the file size is larger in that case.
Gary