I like two fleets (over the course of time).
The first will be the UK fleet which eventually gets moved to sz 6 then to sz 5. I usually have about 4-5 UK trns.
The second would be the US fleet which gets stationed at sz 12. I only need 4-5 trns for the US.
The UK fleet/troops threaten all of Northern Europe, while the US fleet protects Afr for later Japanese landings as well as maintain a threat on SE (even if it is only a bluff).
As the game goes on the US can gradually add more trans and shift troops to Europe or perhaps even create a third fleet and move the one from Sz 12 to Sz 14. Now you can really threaten SE or even Cauc if Russia needs help.
The UK’s job is to get a nice stack in Kar where you can pick off the Germans and/or help reinforce Moscow.
I like to box Germany in then turn the 3 Allies against Japan.
It can be extremely difficult and costly to actually take Berlin using a 1-2 type punch, so I just eliminate them as a threat to Moscow then make sure I can control Japan. After that time is on the Allies side and it doesn’t matter to me if the game goes 10, 15, 25, or 30 rounds because each round that goes by I’m getting more units on the board.