I wouldn't say that there's a simple equation, but it's pretty simple geometry. I threw this together (in about 30min) this evening to illustrate the importance of having the vertical midline of your scope in line with the center of your bore. (Math not included, will add later).

This represents a .30-06 with a 100yrd zero with an 8MOA wide duplex reticle fired at 300yrds where the shooter could hold at the tip of the wide wire on the reticle good for a 4MOA hold over.

The orange dot represents the hold over POA, i.e. what you want to hit. The green dot with "X" on the target represents where you'd actually be hitting.



In the left case, the rifle is held level and the scope is level, such that the vertical mid-line of the scope is in line with the center of the bore. If the rifle is held level, then the arching bullet flight and the vertical mid-line of the scope are co-planar. In short, there bullet flies up and down along the vertical crosshair as it climbs then falls on its way to the target and the shooter can fire at ANY range in a windless condition and only string bullets vertically above and below the target.

In the center case, the rifle is held level, such that the center of the scope is exactly above the center of the bore, except that the scope is rotated 10degrees. As such, when the shooter places the tip of the wide reticle wire onto the target, he's actually moving the muzzle of the rifle LEFT slightly (in this case left). Calling for a 4MOA hold, that slight angle means that his 'crooked correction' would only put him 3.9MOA vertical hold, falling LOW on impact approx. 0.2". The more dramatic problem is that because the reticle is canted, we're now holding over ~0.7MOA windage as well, good for just over 2" of lateral shift. In short, shooter will strike right when holding below the target, say for a 50yrd target, then will strike left when holding over above any target beyond 100yrds. In this 300yrd scenario, that's a 2" left, .2" low strike (same 4MOA vertical hold over using the lower wide reticle wire tip).

Since our eye likes to have things "true," and our shoulder isn't always a great precision indicating tool, it's more common that shooters will slightly cant the rifle to compensate for 'crooked' reticles, as is depicted in the right hand case. In this case, the scope is still canted 10degrees, but the shooter holds the reticle level. The shooter's 100yrd zero is actually compensating for about 1/4" of lateral misalignment, since the barrel is actually starting out to the left of the scope. As the bullet travels another 200yrds passed that 100yrd zero, it'll creep twice as far to the right, good for a bit over 1/2" to the RIGHT of the POA. Since the scope is held perfectly level, the 4MOA hold is correct for vertical correction, so it strikes straight to the right of the target - only 1/2" off.

The middle case is actually very indicative of how important it is to have your rifle/scope level in the field, and why an "anti-cant device" aka scope mounted bubble level is a good investment for a long range hunter.

The more your reticle is canted, the more lateral drift it induces. The farther you shoot, the more drift you'll experience.

Moral of the story: level your scope to your rifle, and always be sure that your rifle is level in the field whenever you're taking a long poke.