For long long range estimates, you can cheat by locking on the target in the uzo and asking the WO for range to nearest visual contact. He gives you a precise estimate up to 30km. If you're going for realism, you can't get range at that distance. Just try to estimate AOB from the diretion of the smoke trail on the smokestack. Maneuver around and charge into position. Get within 5km and start taking stad readings using the 3.15 minute rule. This latter method works just fine for me.
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