Not all planes had totally manual mixture control, for example most US aircraft usually had semi-automatic.
They had full rich for emergencies and start-up when the lever was all the way forward, cut-off to shut the engine down when the lever was all the way back and two intermediate settings: auto lean and auto rich.
In flight, auto lean was mainly used for cruise and fuel economy and auto rich for combat and climbing (richer mixture cools the engine, a good thing to have in prolonged climbs or when using high power settings, even if it costs a bit of performance).
However, not a lot had totally manual control, instead they would just choose between auto rich and auto lean and the engine systems would do the rest.
As for the rest of the aircraft that did have fully manual mixture, it's true that the increments of 20% we currently are not really precise. Leaning properly wouldn't be much trouble even without an exhaust gas temp gauge if the rest of the instruments gave you proper feedback.
If you watch the video in the other thread about prop pitch, you'll see that a constant speed propeller takes a little while to adjust to a new power setting: when you apply more power, the RPMs will rise a little before the prop governor adjusts the pitch to bring them back down, if you reduce power then the RPMs will momentarily drop before the governor brings them back up.
Using that knowledge one can lean the engine by simply watching the RPM gauge. You can lean until the leanest mixture that causes the RPM needle to momentarily jump up. If you go past that into too lean a mixture, power will be less and the RPM will drop a bit, you just enrich a little until it jumps back up again. Essentially, you over-lean once to see what's the peak RPM before the governor can correct it, let it stabilize, then enrich until you reach that momentary peak RPM value again. Also, with a bit of practice it's possible to do it by ear and as previously discussed in a Friday update, it's also possible to judge it from the color of exhaust flames if they are visible.
It's obvious however that for all this to work, we would need some kind of revamp in the way IL2 handles instrument feedback, its sounds and possibly the entire engine model. I don't know how much work that would be and most importantly, with CoD around the corner and the rumors about its improved engine model i'm not sure how much sense it makes to introduce "competing" features in the older series.
Don't get me wrong, i'd really like to have the features you propose, i just don't know if it's possible
