As far as I know (and that is just 1 way to do it), allies must first make a decision: fly the Indian RAF over to Africa or not. If you do it, Egypt is a whole lot better to defend but Calcutta looses its teeth.
If you don’t, Egypt should be lost and Japan will have Calcutta anyway (the RAF will only buy 1 turn respite for India -if at all).
Second, The allies must pay very close attention to (and understand) German build-strategies. Come round 2, Germany will have revealed its targets by positioning but especially by production. If London is not the target, UK should start a FTR-production in SA. And if German support for Italy looks very serious (lots of bomber production), an IC in Egypt is also necessary.
Third, UK must prepare to move its starting FTR/INF into Egypt asap. Experiment with this but you can have all FTR in Egypt by UK3. If stacking two more INF in Egypt will help you keep it, but will cost the UK a TRS, do it.
Last but not least, Russia can help defending Cairo RU3 (landing its aircraft there after attacking Iraq). RU4 they may need to leave again (defending Moscow), but it gives the UK an extra turn of putting 3INF + 2FTR there (max; 3INF produced in Egypt, 2FTR from SA).
Strategic rationale: the combined German Luftwaffe + Italian navy + land units cannot beat this defence. Sure, if they attack, UK + Russia will loose all their aircraft and land units there, but Italy will loose all its land and aircraft units as well and Germany looses as much or even more aircraft than UK + Russia combined. And still this does not give them Cairo. And, after producing a lot of bombers and loosing all but a few over Egypt, the Axis cannot ever hope to take Moscow anymore. In fact, the Red Army could be marching into Eastern Europe by now because so many German luftwaffe is produced and lost.