J3/J4 dow is only if you are industrial bombing russia with japan. Allies have to take as much ground as possible, because russia will fall come G5 or G6, afterwhich usa/uk have to put enough to defend london and egypt.
Japan’s best window is J1 if he wants to attempt a pacific victory.
As allies, there are only a handful of strategies. You can race pacific against europe and go full KJF. You can prepare a massive d day landing off gibraltar, basically build up to take italy / liberate france (you pretty much sit there till your one big attack is good to go). Or you can attempt to turtle up russia/calcutta.
A J3/J4 dow pretty much makes KJF the best bet for allies.