At which exact point are the war assets and galactic readiness evaluated for the ending?

I just finished Mass Effect 3, though I didn't play the multiplayer and my war readiness was therefore at only 50%. So I didn't have enough war assets to get to the best ending.

I now want to play some multiplayer and get that war readiness to a good value, so that I can get the best ending with the same playthrough. I'm wondering now from which point on I need to replay to have my increased effective war asset score count.

At which point in the game is the ending you get determined? When are the war assets counted, the war readiness evaluated and the ending "locked"?

My understanding is that war assets and readiness (and hence EMS) are locked in after Priority: Cerberus Headquarters.

This is based on:

• entering this mission with 7060 assets / 50% readiness did not award the Master and Commander achievement. The ending did not include the final 'secret' scene.
• restarting from Priority: Earth with 80% readiness (over 5000 EMS, which should trigger the 'secret' scene) resulting in exactly the same result as the previous attempt.
• restarting from Priority: Cerberus Headquarters with 100% readiness awarded the Master and Commander achievement. It also resulted in the 'secret' scene in the ending.

EDIT

If asking about Readiness, then according to this Mass Effect Wiki Article:

When the mission Priority: Earth is started, all gathered War Assets combine into two different groups, codenamed Sword and Hammer. Sword consists of all allied fleets and starships, and Hammer consists of all ground forces. When Sword attacks the Reaper fleet above Earth, it acts as a distraction to allow Hammer to land on Earth and engage the Reapers on the ground to aid Admiral Anderson's resistance forces. Depending on the number of War Assets gathered and their combined military effectiveness rating, Sword and Hammer will either take heavy losses during the battle or be able to hold their own. Readiness rating determines if the Crucible, depending on the solution chosen, destroys all forces fighting on Earth indiscriminately, or selectively targets the Reapers.

So the answer for War Assets is right before the Priority: Cerberus Headquarters (the point of no return).

While Galactic Readiness waits until Priority Earth (the actual final mission

I would venture a guess is that when you hit the point of no return, i.e., right before the assault on the Cerberus headquarters, your War Assets are then calculated for your ending. So in order to get the best ending, ensure you have as high a rating as possible in order to gain the ending you desire.