When the recline adjustment on the base is not enough, a common solution is to use a rolled towel or pool noodle. Some manufacturers want the foot on the base fully extended when you are using it with a noodle; some manufacturers want the foot on the base fully retracted; a few don't allow any noodles at all. Your manual says to fully extend the foot, so that's what you should do.
Pool noodles, tightly rolled towels, or a rolled up sweatshirt, it doesn't really matter as long as the item is lightweight and relatively non-compressible. Lightweight so that it is not dangerous as a projectile, and non-compressible because it wouldn't do much good if it just squished away when you tighten the belt.