martin Many people have 3 degrees deviation (50%), some up to 5 or 6. I have 20 (in horizontal). I think from certain amount, the issue starts to be instant.
It would be extremely interesting to know if there are people out there with pronounced heterophoria, who do not suffer of extreme and immediate eyestrain as we do. As well as if there are people with no heterophoria who share our "luck" of being so sensitive to modern technology.
I personally suspect something more than just heterophoria, something on the neurological side, because my literally instantaneous disturbs are not due to any kind of fatigue, they occur on the spot.
That said, I am not a medical doctor and @martin I think it is great you are getting treated, and I am not surprised your condition and your tolerance are improving. I am too feeling not as awful as a few weeks ago, after I restarted doing convergence push-ups, Tibetan wheel and other basic exercises for eye muscles.