You identified one of reasons why it is foolish to use a wide treadle step on a chicken feeder. Ideally you want the hen a bit stretched out, standing on one foot, using the other foot to hold the treadle down. She isn't comfortable enough to stand all day like that raking feed. She won't be able to perch on the 1.5" wide treadle bar and lean forward, she would tip over, so she is forced to put one leg forward to balance herself. The other reason is swarming of rats or wild birds like pigeons that are heavy enough to over come the pre load on the door, assuming that the feeder even has springs to pre load the door as most treadle feeders do not. You want that treadle far enough back that it would take enormous cooperation between a dozen rats or pigeons to hold the door open and feed. At some point the ones at the back will rush forward and the door closes or the rats fight their way inside and get their dirty *sses smothered to death. That is rare, usually only in commercial flocks with hundreds if not thousands of rats already present. But once is all it takes, the rats will not touch a feeder that has killed other rats.
Another thing you can do if your feeder is designed for it is to use a feeder lip extender. There should be a 1/2" wide lip extending into the feed tray to prevent raking, of course the Chinese made feeders like the Grandpa and its many clones do not have that feature. They do put a wire grid on top the feed, a poor stop gap measure at best. So if you can find a short piece of 1/2" x 1/2" aluminum angle pop rivet it or screw it in place with sheet metal self tapping screws, on the inside of course.
If you have a proper feeder with enclosed sides and an inward swinging door you can use the feeder lip extender which is a 1.5" tall strip of sheet metal that slips between the side flanges and the front panel, extending the height up about 3/4" of an inch. Not many chickens are going to rake feed past that much of a lip. If you have a hen from hell you might have to cut some chicken wire and lay on top the feed, let the hens peck feed out a beak full at a time.
One last tip, make sure your feeder is rock solid, zero movement at all, to prevent shaking too much feed down into the lower tray. In an ideal set up, there will be a thin layer of feed flowing down the angled feed tray bottom. It is self supporting with the angle of repose for the feed in mind, allowing a small pile up of feed at the bottom that actually prevents more feed from flowing down. Unless the feeder is wobbly, allowing more vibrations from the lid slamming or chickens jumping on the treadle to shake down too much feed. So secure the feeder to a wall or post and have a solid patio block base, which you seem to have done. Or place a heavy patio block on top the feeder to help secure it. If nothing else, choke off a portion of the throat so the feed doesn't flow as heavy. Cardboard works well, play with the amount of throat blocked off until it works for you.
