Although it is certainly true that a rifescope will usually need to be mounted much farther forward (toward the muzzle) for shooting prone than it will for shooting standing or supported from a bench, where exactly the rear of the ocular winds up depends on a number of factors such as shooter anatomy, eye relief of the particular rifle scope, and the length of pull (LOP) of the stock.
I have scopes mounted on two Ruger 10/22s. One is a 3-9x40 and the other a 4x32. One 10/22 is mounted on a Magpul Hunter X22 stock and the other a Ruger wooden carbine stock. Although I think my neck length is relatively proportionate to the rest of my body size, my arms seem to be about 1 1/2" longer than would be expected for my height and torso length. The LOP of a typical Ruger carbine stock is too short for me.
On the Magpul X22 stock, I use all of the stock spacers. The stock usually comes with two of the four installed. The additional 2 spacers makes the LOP 2 inches longer at 14.5". With the standard Ruger carbine wooden stock I have fitted a John Masen recoil pad that increases the LOP to 14.5":
http://www.midwayusa.com/product/10...ni-14-and-mini-30-rifle-extended-rubber-black
Needless to say, with the increased LOP when I drop my cheek naturally down on the stock, it winds up being an inch farther back on the comb than it would for a 13.5" LOP. On my wooden stock Ruger, the Beartooth comb riser kit works fine for that positioning. On my Magpul Hunter X22 stock, the shortest comb riser suffices.
I position my scopes based on the prone shooting position which for me is the least forgiving when it comes to fore and aft scope position. I place the ocular of the scope just far enough forward that I have a full field of view with no vignetting at the highest magnification I intend to use. For me, this positions the ocular of the scopes on both my 10/22s almost exactly 1" behind the rear edge of the receiver, the amount by which I have increased the LOP of the stock.
Whether or not you need to mount your scope farther forward than straight up rings allow depends a bit on the scope dimensions as well as its eye relief. With some compact scopes with a short ocular bell and a short rear tube, you might possibly get by with vertical rings. But the stock Ruger accessory rail will probably not allow most scopes to be mounted correctly with straight vertical rings. Although one-piece cantilever scope mounts are very nice, they tend to be pricey, and there are several cheaper options that will work.
One is an extended Picatinny rail that fits the Ruger 10/22 receiver. Evolution Gun Works makes a very nice one:
http://www.amazon.com/Ruger-10-22-Picatinny-Scope-Mount-x/dp/B004VIANJA
This is a true Picatinny rail that will work with all Picatinny and Weaver spec rings and has more mounting slot options than the stock Ruger Weaver/dovetail accessory rail. The front of this rail will extend right up to the rear of the Ruger stock rear sight leaf when it is flipped up, but you will not be able to use the stock Ruger sights with it as the rail itself will obscure your sight picture. But it will allow you to mount nearly any riflescope in the proper position using straight-up, vertical rings.
There are also two-piece ring sets with one extension ring and one vertical ring. I have used these Weaver rings with good results:
http://www.ebay.com/itm/Weaver-4904...h=item489e7387e6:g:lWQAAOSwGPxaBjwh:rk:7:pf:0
These are sometimes found on ebay for just under $13 a set. Although the ring height is described by Weaver as "high" the saddle height for these rings (top surface of rail to bottom of scope tube) is actually only .332", just barely high enough to allow me to mount my 3-9x40 scope. But the forward extension on the extension ring is less than 1" which is just enough for me to properly mount that scope with its ocular 1" behind the receiver body. It may not be enough extension for many.
Hawke makes similar sets of extension rings with either a 1" or a 2" forward extension:
http://www.opticsplanet.com/hawke-sport-optics-2pc-1in-weaver-high-2in-extension-rings.html
These have a higher saddle mounting height than the Weaver Quad Lock rails. The 2" extension model should allow virtually any riflescope to be mounted on the stock Ruger accessory rail.
My solution to trying to achieve a consistent cheek weld is to position my head just far enough forward to get a complete field of view through my scopes without vignetting. As I said before, the scopes are positioned just far enough forward to provide for that when my head is positioned with a natural cheek weld when shooting prone. When shooting sitting, I need to "turkey neck" my head forward just slightly to achieve the same head position. When shooting standing, I need to turkey neck forward quite a bit more.
I have not shot using a stock with an adjustable LOP. If I had one, I suspect I would shorten the LOP somewhat when shooting offhand standing.
