Cast-Off (Right hand shooter ) is the amt that the butt plate is displaced to the right of the center line of the butt of the stock.
That centerline of the butt is in line with the bore of the rifle.
A straight line can be drawn from the tang of the action in line w/ the bore,,all the way to the heel of the stock along the comb, then down the flat of the butt of the stock.

Placing the butt plate on that line on the butt portion of the stock gives you no Cast -Off.

Move the butt plate any amt to the right of that line and you have Cast Off for a right hand shooter.
CastOff allows many (not every one) to more easily sight the gun, to get their eye in place comfortably behind the sights.
The rifles sights,,your right eye, your right shoulder, your face/cheek against the stock are trying to align and do it comfortably

(You can alter the Cast off even more to add 'Cant' or called Kick-out' sometimes.
The bottom screw hole is displaced a bit further out from the center line than the top. The butt plate is Canted. This is purposely done to make the rifle more comfortable to shoot (for some people) as the shoulder is not a straight up and down joint.
You will see this adjustment on many Brit gunstocks)

With the butt plate mounted with the amt of Cast-off you desire and any or no Cant to it. The center line of the stock now moves slightly to the right along the comb to meet the new location of the center of the butt plate. However it still begins at the same point up front at the action tang where it aligns with the bore center.
The Toe line of the stock is drawn using the bottom center of the butt plate and the center of the grip (cap).

Now the stock is shaped to those new center line(s). The same shape as if it had no Cast-Off,,just that the butt stock is moved over to the right a slight amt and if the Cant is added the toe is kicked out a little.
Adjustments in shaping are done for these changes but they are not obvious to the eye unless you glance down the stock from either end. Easier to see if you turn the gun upside down and look from the bbl end actually.

A cheek piece should be no different, larger or smaller than if there were no cast off at all. It's just displaced a very small amt to the right because the comb line center moves that way tapering from a tiny amt at the nose to the full cast off amt at the heel.