From: Utlima Thule
I'll try and give you my answer, but all this is very conditional, not least on player skill and so on.
Don't mess with the core settings, keep them at 100%. I've just had a potentially great game that had lasted 15 months (real time) fade into the ground due to the unintended impact of changing this.
Think about what is 'historical' in your eyes. There are posters here who see nothing wrong in getting Pzrs to Stalingrad by T15, so its a pretty broad concept. To me, a historical game has an ebb and flow. Rampant German offensives June-July 1941, stiffening Soviet resistance Aug-Sept 1941, final round of German attacks late Autumn, a Soviet counterattack that takes the Germans to the brink but can't quite fulfill its potential, a German attack with very similar characteristics, late 42-mid 43 some sort of brutal attritional stalemate, and then mounting Soviet dominance?
Let the +1 go on attack (and don't use the +1 on defense, thats something I'd give the Soviet AI), it'll mean less attacks in 1941 but not critical. Ideally with no +1 you need a blizzard setting somewhere between the current 'reduced' and the old 'excessive'. If you feel you are well matched, you've just got to pick one or the other, if you feel one of you knows the game better etc, that might be a good balance rule. The severity will play very directly into how dangerous the Germans are in the Summer of 1942.
Keep airborne, but agree a limit. My last PBEM we had none before Nov 41 (which is realistic, there is no way the Soviets could have organised one), one a month to June 42 (but you can save them, so if you don't drop in Nov 41 you can do 2 in Dec etc). This stops spam, but makes a German player have to think about rear area security.
Use random weather - this will favour the Soviets in 41 and the Germans for the rest of the game.
Try to not do HQ bombing - you can't completely avoid this as limited recon may only tell you there is a unit in the hex. Not for the old reason of avoiding silly levels of leader death, but instead its just too effective at destroying support units.
Keep to a limit on bombing airbases, or the Soviets will simply overwhelm the Luftwaffe over time.
HQBU is a problem. I really don't like it, its completely artificial (though 1.08 has done a lot of good), but without it the Germans will stall too early in 1941 and it will remove a lot of uncertainty for the Soviet side. Also too hard to police in PBEM, so let it be, its a bad means to a good end.
Consider a trade off in the Ukraine. No Lvov pocket, Germans to stay north of the line that releases S Front on T1. In turn, no Soviet mass evacuation to fill out the defense of Leningrad. In combination this makes the Ukraine a lot more fun for both sides than the alternative of the good Soviet units heading north and the Germans having to wade through a sea of 1-1 ants laid out like you are playing checkers.