The Percentage agreement isn’t of any great significance in this situation; the Greek Communists had all the resources they needed to win control of the country with the withdrawal of the Germans, courtesy of the British who had been supplying them for most of two years.
The SHAEF and Washington were both bitterly opposed to any interference in the internal affairs of liberated nations and equally opposed to any diversion of forces away from the task of winning the war and towards what they (in some ways rightly) saw as British imperialistic meddling. The Americans refused to provide amphibious landing craft to transport British troops to Greece and protested loudly the diversion of British troops that were meant to be boosting Alexander’s forces in Italy; the British had to use other means. Had Eisenhower and Washington been more insistent and Churchill been more conscious that he risked alienating Roosevelt before the Yalta conference, where the stakes were far greater, literally the fate of all of Europe, he may have been more reluctant to authorise force against the KKE.
If the Communists been slightly faster seizing control as the Germans drove north, or the British been a little slower a scraping together what little forces they could spare, the Partisan flag would have been flying over the Acropolis and the world would have been presented with a fait accompli.
Had the Communists come to power in 1944 the Balkans would have had two (and later three) Communist, but not Warsaw Pact Member, nations. Siantos’ self-liberated Greek Communists would have been no happier to welcome the Red Army and Moscow’s domination than Tito was.
This would have affected the independence movement in Cyprus; the last thing the Greek’s there would have wanted was union with a Communist mainland.