My natural shouldering of rifle is cantering slightly inward. Thus, the scope is compensated by using a vertical line 40 foot away on the basement wall for proper cross hair vertical adjustment. Do you find this acceptable? How do you do it, then?

I've seen a few folks that have a real cant when they shoulder their rifle. Those folks set there scope so it's verticle with their hold, that will keep your scope tracking (right/left, up/down)easier when you need to make a change.

Yep.. seen lots of great shooters who do that. I'm not one of them, but my scope is canted about three degrees off vertical from the center line of my rifle due to my canting the rifle slightly inward like that in position. The most can't I've seen in a rifle was used by Dave Imas. I had the pleasure of shooting his rifle a few times and I felt like I was holding the rifle like a sideways "thug style" pistol hold just to get the vertical crosshair back to vertical.

Now here is a thought on this. If your scope is directly above the c/l of the bore all you should have to worry about is the drop of the bullet. So changing elevation between animals should be all that is needed.
If your rifle is canted that will put the scope to the left or right of the verticle center line of the bore. With your rifle canted and zero set at 100 yards. Looking from above now imagine two lines. One from the center line of the bore and one from the c/l of the scope, both going to a point on a target 100 yards away. Two seperate lines that meet at the target. If you took these lines further to lets say 200 or 300 or 400 yards you would see that they crossed at 100 yards then started seperating as they went to 200 and on. So if you cant your rifle be prepared to see not only a verticle but also a horizontal change as you adjust your scope settings from one distance to another.
Now, if your cant is 1/4 inch off you do the math from 40 to 100 M Not much 200 to 500m would be a bit more.
We saw this with the chin guns back in the 90's with the offset scopes. You had elevation zeros and windage zeros but if I remember right in rimfire you could pick a point at around pigs for your windage zero and you would see very little change from 40 to 100 M maybe 1 or 2 clicks.

Yankee,

Thanks for bringing this topic up. It is near and dear to my heart after almost 14 years of focusing on Black Powder Cartridge Rifle matches: silhouette, mid-range (200, 300 & 600 yards,) long-range (800, 900 & 1,000 yards,) as well as gong (buffalo) matches out to 1,235 yards. Because of the MV's and projectiles used in the BPCR matches, the issue you talk about in your post is considerably more important in those types of matches; and, must be taken into account if one wants to "run with the big dogs."

We have two phenomena that must be comprehended and compensated for when canting a rifle. One is the phenomenon you posted about; the horizontal offset between the aiming dot or crosshair-center and the center-line of the bore. That analysis is simple and straight forward with just a high school geometry class background. Let's go through that analysis first.

I cant all of my rifles: AirRifle, SB, HP, ATC and BPCR rifles when shooting offhand. My SB and HP silhouette rifles have about 3/8ths of an inch off-set to the left, me being a right-hand shooter. And, I might add, that is quite a bit of cant, according to others that have shouldered my rifles over the years. I don't know if it is as much as Dave "The Leaning Tower of Pisa" Imas'; but, it is still substantial. Anyway, when my canted, HP rifle is sighted-in at 200 meters, the offset for the POI, to the right, at rams, is only 0.78"; not much, eh??? Now if the rifle is windage-zeroed at 300 meters, the ram offset, POI, is only 0.52". Who can hold the difference? The bullet's precession, or "spin drift," also known as "gyroscopic drift" is more substantial. You guys that ride motorcycles at high speed around corners know all about "wheel precession."

I'll run the precession numbers...back in a Flash, Jumping Jack...OK...for a 162 A-Max launched to 2,400 fps, the spin-drift, if the rifle is sighted-in at 200 meters, at the ram line is 1.9" to the right of POA (point of aim.) That dwarfs the "cant induced" offset which is only 0.78", or about 0.136 MOA. Oh, I might add, they are additive; that is, sum the two effects for the total effect.

The short admonishment is; zero the rifle, windage-wise, at either the pig or turkey line in calm conditions and fo-getta-bout-it if one has "cant" concerns. I've done a bunch of testing to determine how to figure-out rear-sight-cant-angles for BPCR rifles, that are way into the extreme range concerning this stuff and can help the HP and SB shooters getter'done. That is, how can we do this offset adjustment at just 100 yards, in calm conditions, and still nail it out to 500 meters or even 1,000 yards??? It is doable, maybe later, if the muse so motivates me, or is that, metastasizes in my brain.