How to slow down Soviets without EF Box on?

[Lovenought]

I'm interested in slightly modifying a scenario to allow the game to run to August 1945 without requiring the Axis player to manage the headache that is the EFB.

I know it is possible to change the rate at which the Eastern Front advances with EF Box off, since in the 4th Supreme Command the East Front timetable is delayed by a month. So clearly it's possible to do it somehow. But I want to be certain I've figured out the actual proper way to do it.

In the scenario editor there is an option to change the "End at start of turn" number. Which starts at 101, in 4th Supreme Command. Will increasing this number actually result in Berlin falling to the Soviets at a later time? I get the feeling it won't....there is one other value that looks promising. "Initial Soviet Potential". But it can only be set to -50 at max.
Does this value even have any effect on East Front progress when EF Box is off? And if so, how many extra turns would the Germans get even with -50?