The distance when the engage begins on Sh3 depends so much on weather factor. In stock Sh3/GW/NYGM/RUB the maximum distance was 5-6Km and the medium one is 3KM (weather factor limitations).
But wait for the upcoming GWX...battles will start at maximum at 30KM!! and medium ones at 10KM!! It's very nice. We also have now all the main historical WWII Ops on GWX. Really a lot of them, far more than any mod ever.
Ops...I already said so much. More on this only when GWX come alive.