Progressive lenses have the distance (if any) prescription in the top half of the lens, the close (reading) prescription near the bottom, and a continuous change between the middle and bottom. When my reading prescription was weaker, the continuity was more gradual; by tilting my head I could change the power so that the front sight on my handguns and long guns was in exact focus. Now that my reading prescription is stronger, the transition is more abrupt, and I cannot find the sweet spot. An older pair of (weaker) reading glasses will work for non-optical sights.
When welding, the lower portion of the progressive bifocals is blocked by the helmet. I use a pair of reading glasses that is now stronger than commonly available cheaters and gives me "full" field of vision.