astigmatisim II

If you have astigmatism is what you are aiming at actually in that place?  Too many factors to assign this to one item; parallax is the most common item, the quality of the scope, alignment of the scope to the bore centerline, etc.

Where the cross hairs are pointed, is that where the bullseye is? - Not necessarily. If your eye is in the center of the scope and the scope is adjusted for parallax at that range, you are looking straight at the target.

Or does the correction in your glasses compensate for any error in perceived vs actual location? However small this is. -  No. See the note at the bottom.

Also if you change your head position does the aiming point also change? - if you are not centered and the scope is not adjusted for parallax at that range, you are moving off-target when you move your head.

Our corrective lenses put the center point of our prescription in the center of the lens and we (me) actually aim through a peripheral part of the lens closer to our nose. - Yes, I've just had an eye exam with an ophthalmologist and we discussed this at length. I have two prescriptions' for glasses, shooting and normal reading. The shooting glasses prescription is focused on the front sight, where your concentration should be. I've had cataract surgery and can see everything well beyond one foot in front of me.

How about with a peep sight? - Same as the scope. you concentrate on the front sight. In a scope, you concentrate on the crosshairs. Put a scoped rifle solidly on a bench with 100 and 200 yard targets, then look critically at the crosshairs and the target. Each will appear slightly fuzzy when concentrating on the other. Seeing the target is so much easier in a scope, shooters forget to look at the crosshairs. 

A friend of mine cannot hit a duck with his glasses on and slays them just fine sans glasses. - Ric is correct, eye dominance.  I am left-handed, but right eye dominant; I shoot right handed.

I already know I have more trouble with floaters, always have. - Common problem in people, starting in middle age. Alice has such a problem that she cannot shoot a dot scope, she gets confused. She has a Weaver T-36 with crosshairs that are thick enough to not lose in the clutter.

Note #1: I've tried Aviator style glasses before thinking the deeper glass will have clearer vision when the head is tilted, but this did not work. I now have the biggest frame with the deepest lens in "normal" glass frames. The partition on the bifocal is low, set to allow my eye to look at the scope or the irons, the lower part is narrow to allow reading the scope turrets adjusting for range. I'm adjusting the scope height in the rings and stock to where my head is more erect and the eye more centered. For benchrest competition, this is easy to do. For prairie dog shooting, this is not feasible since I shoot mostly Ruger #1's with their factory stocks.

Note #2: An easy way to sight a scope without parallax adjustment is to cut a center hole in a target paster, stick it on the objective and look through the small hole. Did this often with Weaver K4 and K6 scopes.

Note #3: While the scope price is an indication of quality, you can work with and understand the limitations of the scope you are using. All scopes get fuzzy on the edge, the more expensive scopes have a much smaller fuzzy ring. {I love the external adjustment and Redfield 3200 scopes. They have their challenges.)