In regards to the gunsight, there's a unit of angle measurement called mil (just think of it as a very small sub-division of a degree) that is used to categorize them and it's all based on a very simple fact: the apparent size of a target is inversely proportional to it's range. In other words, if a plane with a wingspan of X meters appears to be Z mils, then the same plane at twice the distance (2X meters) will appear to be half the amount of mils (0.5Z mils).
The Revi gunsight used in the 109 is a 100 mils sight. That means that a target with a wingspan of 10 meters will have its wingtips "touching" the bars when it's 100 meters away from you (just like you said in your post).
According to Flea's checklists (see the sticky threads in the general section of the cliffs of Dover forum) the Spitfire and Hurricane have about 11-12 meters of wingspan, which puts them at around 110-120 meters when their wings fit between the horizontal gunsight bars.
For other distances you need to extrapolate. I think that each horizontal bar is the same length as the empty space between bars and also that the circle "cuts" each bar right in the middle. If this is true, then the following applies:
span fitting on top of one whole bar = span fitting between bars = a range of 110-120 meters
span fitting on top of half a bar = span being half the distance between bars = half the apparent size and thus double the distance (220-240 meters)
You will either need to judge this by eye, or kick a bit of rudder to momentarily skid your plane and align the gunsight bar with the target's wings to take a quick range reading.
For aircraft with different wingspans it needs further calculations, but in that case i just roughly approximate by assuming that a medium twin-engined bomber like the Blenheim should be about 1.5 times a fighter's size and a bigger one like the Wellington should be roughly twice the wingspan of a fighter.
For example, for a target 2x the wingspan of a fighter his apparent wingspan would be twice the space between bars when viewed from the "default" 110-120 meter range. Or inversely, the range would be twice the "default" range when he fills the space between bars ( = 220 to 240 meters).
If calculating this "on the fly" is driving you nuts, you might prefer to just memorize the list of ranges when "touching" the bars for known aircraft, or print it out on a small strip of paper and tape it to the side of your monitor/PC case/desk/etc.
Start by downloading Flea's excellent checklists from the sticky thread mentioned before. In there you'll find the range at which every allied aircraft in the sim fits between the Revi's bars.
From that point on you can extrapolate easily: if a certain aircraft type is at X meters when it touches the bars, then it's at 2x meters when it's half a bar long, 0.5X meters when it's twice the space between bars and so on.
It's pretty obvious that it's impossible to get a 100% accurate ranging in the heat of battle against a maneuvering target, except when attacking targets that pretty much fly straight and level (like bombers). In any case, fighters take less hits to go down so it pretty much evens out.
So, now that we know how to measure range to target, when do we shoot?
The answer is "at the range where the rounds of all your guns are harmonized to meet", in other words your convergence setting. I think the default one is 333 meters, however you can go into the loadout screen and modify this if you want to (loadouts are somewhat bugged, see the sticky FAQ thread in the general section for more details on how to successfully set them).
This means that you will either have to run some further calculations and get an approximate apparent size for most enemy aircraft at the range your guns converge at and fire when they are at that range, or inversely, you can set your convergence to a number that produces an easy to remember/calculate gunsight picture.
For the 100 mils Revi, that would mean mostly setting it in even, whole hundreds of meters (100,200,300) instead of in-between values (like 125,275 and so on).
In any case, you can start with an easy to use setting and as you gain experience you'll see it will be much easier to gauge distances without having to place the gunsight on the target but just by looking at the target, which means you can then start to use uneven numbers for convergence to suit your tactical needs better.
In other words, at some point you start developing a feel for all the parameters (relevant speed between you and the target, amount of deflection, the ballistic characteristics of your guns) and you can shoot and score hits from weird angles just by judging the amount of distance between the center of the gunsight in relation to these parameters.
For example, i use default convergence for the 109s cowl mounted machine guns so that they are accurate at long range. I use these to score some hits on enemy fighters and get them to turn and bleed off some speed, allowing me to close the distance quicker. I fire a few more bursts with the machine guns as i keep closing and sometimes this is all that's needed to cripple or shoot down the target.
If he survives i keep closing in on him and use my cannons, which i have harmonized to 100 meters for delivering the killing blows: their ballistics are not that good and their ammo supply is low, so it makes sense to use them at close range where it's more probable for the rounds to connect.
If i had them harmonized to the same distance as my machine guns and i was shooting from 100 meters away, my cannon rounds would be going both wide and high of the target (i would have to aim lower and at least one or both of each cannon rounds would still be passing harmlessly outside the target's wingspan), but setting them for close range gets rid of that problem.
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