Well, generally speaking most gunsights are calibrated for a certain wingspan. Don't be surprised for example if the Revis are calibrated for Spitfires.
Now, the way this works is that if you know the approximate wingspan of your targets and the winspan of the target the Revi is calibrated for, you can make some quick calculations and deduce range.
For example, let's say we have a gunsight that's calibrated for a target with a nice, easy, wingspan of 10 meters. The other part of the calibration is at what range the sight is calibrated at. So, in order to make it easy once again, let's say it's calibrated for a target wingspan of 10m at a range of 100m.
What this means in simple temrs it that a plane with a 10m wingspan will fill the entire gunsight when it's 100m from you. I think that by "fill the gunsight" , most gunsights usually mean the circle and not the 4 lines/cross that extend past it. Now, if we see that the same target with a wingspan of 10m fills about half the circle, we know he's 200m away, if he fills a quarter of the circle he's 400m away and so on.
Conversely, when the target has a bigger wingspan it goes the other way around, ie we need to first assess what kind of "sight picture" we'll be getting at 100m for our new target or alternatively, at what range the new target will fill the entire gunsight, before we start approximating the rest of the ranges.
As an easy example, let's say the target is a twin engined bomber with a wingspan of 20m. It's double the span of our original fighter sized target, so it will show up double the size at the same range--->only half of the bomber's wingspan will fit in the gunsight at 100m, but at 200m it will fit nicely in the gunsight's circle. So, when it fills half the gunsight he's not 200m away as the fighter war, he's 400m away.
In short, for 10m span/100m calibration:
target wingspan-----------sight picture---------------range
fighter, 10m-------------------entire sight-----------------100m
---------------------------------1/2----------------------200m
---------------------------------1/4----------------------400m
bomber, 20m-------------------entire sight----------------200m
---------------------------------1/2----------------------400m
---------------------------------1/4----------------------800m
After looking around, it seems that the Revi (at least in IL2, don't know about the real one) is calibrated so that a Spitfire at 100m will fill the entire gunsight. Don't ask me which mark, clipped wing or not
Wikipedia gives a wing span of 11.23m for a Mk.Vb.
So, i'll just go on a rough but good enough approximation here and just go ahead and assume that 10m of wing span when viewed from a Revi at a distance of 100m, will give us a sight picture of the target's wingtips being slightly inside the gunsight's circle.
Better yet, we could look up the wingspans of the most likely targets. We already know that Spit(11.23m) at 100m will fill the gunsight.
A P-47D is a bit wider at 12.42m, so at 100m range his wingtips will be a bit outside the gunsight's ring.
A P-51D is very close to the Spitfire, with 11.28m of wingspan, so we'll just treat it the same (100m when gunsight is filled).
The A-20 is 18.69m. Let's make it easy on ourselves and say it's almost roughly 1.6 times the Spit's wing span, so when it fills the gunsight it will be at 160 meters.
A B-25 is 20.6m, so it's almost double the size of the Spit. That means it's almost 200m away when it fills the sight.
A B-24 is 33.5m, that's about 3 times the Spit's size, so it gives us 300 meters when the sight is full.
A B-17 is 31.52m, so it will be a bit less than 300 meters away from us when it fills the gunsight circle.
That's the first part of using the gunsight, so that you can determine range effectively. There are other more technical issues as well, but i'm not as familiar with them. For example, you can use the sight in accordance with your angle-off target to determine the lead you need to pull. However, this can get complicated depending on relative speeds between you and the target, as well as the projectile ballistics.
Most of all, in planes like the 110 that carry 2-3 different kinds of guns, the convergence might seem not to be an issue but because of different ballistics the guns have different arcs in the vertical axis. Shooting a bit outside of convergence you might sometimes see that while the gunfire "meets" on the horizontal axis, there's still vertical separation between the shells, ie you could be getting hits on the target's fuselage, wingroots and wingtips, but half your cannons are shooting too high or too low and you waste half of your projectiles.
This is not so much a problem when you have a lot of ammo that still deals a good punch however.
I'm not very experienced with estimating lead from angle-off with the gunsight so i usually do it by experience alone, but i think you can find a good tutorial on the movies section of this forum (it's called "bag the Hun" i think) that discusses a few more technical stuff:
http://forum.1cpublishing.eu/showthread.php?t=5012