Yes, I checked on this and am sure that it is PrestigeOfDeployedCoreUnits / NumberOfDEPLOYABLEUnitsThere is a confusion about how the average unit value for the softcap is exactly calculated. It's either PrestigeOfDeployedCoreUnits / NumberOfDEPLOYABLEUnits or PrestigeOfDeployedCoreUnits / NumberOfDEPLOYEDUnits.
So, deploying fewer units helps toward the soft cap. I considered this, but chose rather to deploy some cheap units, e.g. StugIIIB or the SdKfZ 7/1. Both with a high RoF, the Stug with HA of 8 can be of great help even late on; I converted a Range hero Arty into a Stug which I kept until the very end in Berlin. Unfortunately, that one was my only Range hero arty, otherwise I would have repeated this move.
And the SdKfZ 7/1, while one point down in AA has that high RoF compared to the 7/2, and is 50% cheaper than the late SPADs. This one too I used all the way down to Berlin 45.