Thanks for the link. I've read many of that guy's posts on various forums and he seems to know what he's talking about.
He says that the optimal place to focus is the hyperfocal distance, which is twice the distance from your eye to the front sight, if I remember correctly. Apparently this results in getting both the front sight and the target just within your depth of field. And that's why he recommends around +0.5 or +0.75 diopters - this puts your natural focal point at around 2m from your eye, which is about right for most rifles. My prescription does actually include +0.5 (right) and +0.75 (left) of spherical correction, and I didn't understand this at the time, but maybe that's why the doc included that. But I am still trying to focus on the front sight, and maybe that is what's causing my problem. Maybe I should practice differently, although I'm not sure exactly how.