Correct, adding length ( washers) to the top should reduce upward jump ( which is where the face slap comes from) and will redirect this energy rearward, slightly increasing felt recoil at the shoulder. This might not even be noticeable because the contact patch on your shoulder is quite large and a recoil pad softens the blow. The washer trick of course is just temporary to test the theory, a few shots or more probably a round of 25 or two should tell you if you're on the right track. As mentioned, if this is the magic formula to reducing your face slap and doesn't introduce other issues, your next step will be a permanent adjustment to your butt (stock). This can be an angled cut with a new suitably thick recoil pad or one of several adjustable recoil pads which usually can adjust for length of pull, drop at heel, pitch, and toe in or out. More expensive, more versatile, more popular on more expensive guns. And as already pointed out , pitch may not be your problem, or only part of the problem, but this test should give you some idea of where pitch fits into the picture without altering your gun or spending money. You mentioned that the top of the butt pad protrudes above your shoulder and if you do this to get your proper sight picture you need an adjustment in drop at heel. This wouldn't likely be causing your face slap by itself but it may be contributing by causing you to compromise your hold. The adjustable pad will allow you to lower the butt pad as well as alter the pitch.