The sight reticle is currently defaulted at...what? fighters? Wingtip-to-wingtip. Anyone know how to do better than eyeballing when you adjust the size of the reticle?

You must fire your weapons only at the ranges for which they were designed. Normally, you should not fire at ranges greater than 400m, as beyond this distance the trajectory quickly falls away. This also applies to larger weapons such as the Mk 108. It is a common fallacy to believe that it must be possible to fire larger-caliber weapons at greater ranges, and that the aiming of these need not be as precise as for small-caliber weapons. The opposite is true! You have little ammunition! Use it sparingly. With the Mk 108 you only have 60 rounds, so reduce range, aim carefully, fire accurately.

Commence firing at a maximum range of 400m. Experience has shown that combat ranges are being greatly underestimated. The distance given in combat reports are almost never correct. When, for example, a combat range of 50-100m is reported, it actually varies between 200m and 400m, as established by analysis of combat films. Frequently, range estimation errors are even greater. In attacks against bombers, many fighters open fire at 2,500-3,000m. This is a senseless waste of ammunition!

A four-engined bomber has a wingspan of about 30M. If it appears a large as the deflection circle it is at a 300m range. (I set mine up for 400m so you might want to change the multipliers) For a four-engined bomber, the estimated value corresponding to a span of 30m is multiplied by 3. Therefore, a four-engined bomber with 30 m span fits in the circle one time - 1 x 3 = 300 m; fits in the circle two times - 2 x 3 = 600 m, fits in the circle three times - 3 x 3 = 900 m.

July 1944 Gunnery Instruction Document

You must fire your weapons only at the ranged for which they were designed

Afaik the sight is supposed to be adjustable but it defaults to either 250 or 300 yards for a fighter sized target (Wingspan approx 32ft). Given that your closure rate will often be rapid (being the skilled, tactically superior aviator you are) there's not much point in picking bones about a mere 50ft since you'll only be in convergence range for a limited time. When aiming at bombers, place the circumference outboard of the engines and you won't be far off. For Bf-110's estimate your aim as the length from one wingtip to the outboard engine nacelle and you'll be on the money.

(Provided that the default range is observed by WoTR which I assume it is.)

I hope that adjustable (and gyro) sights become a thing for WoTR, 300 yards is a long way to shoot at high speed.