I can easily see stereographic images where you have to focus on a plane in front of the picture, but with this implementation you should focus behind the picture. That's hard for me to do, and this implementation doesn't allow this to be toggled.
You can focus in front if you wish, the depth is just reversed but that doesn't matter, may be easier if you zoom in the page. I got 4860 focusing behind
Zooming helped me as well, it allows you to fill more of your vision which I find helps.