+ Show Spoiler [how EMS affects the endings] +
Straight from the Collector's Edition from the guide (I will type this nearly word for word, credit goes to Prima Games):
EDIT: The game assumes if you do not import a save then you DESTROYED the Collector's Base
If you saved the Collector's Base in Mass Effect 2, then:
If your Readiness Rating (War Assets multiplied by Galactic Readiness, will be called EMS from here on out) is below 1750, Earth is destroyed regardless of your final choice.
If your EMS is 1751-2049, if you choose to destroy the Reapers, Earth is also destroyed.
If your EMS is 2050-2349, if you choose to control the Reapers, Earth is saved.
If your EMS is 2350- 2649, if you choose to destroy the Reapers, Earth is devastated but still saved.
If your EMS is 2650-2799, you unlock the third (Synergy) ending.
If your EMS is 4000-4999, if you choose to destroy the Reapers and "saved" Anderson, Shepard lives.
If your EMS is 5000+, if you choose to destroy the Reapers and did not "save" Anderson, Shepard lives.
Saving Anderson means not letting the Illusive Man kill him outright.
_____
If you did not save the Collector's Base in Mass Effect 2:
If your Readiness Rating (War Assets multiplied by Galactic Readiness, will be called EMS from here on out) is below 1750, Earth is destroyed regardless of your final choice.
If your EMS is 1751-1899, if you choose to control the Reapers, Earth is also destroyed.
If your EMS is 1900-2349, if you choose to control the Reapers, Earth is saved but devastated.
If your EMS is 2350- 2649, if you choose to control the Reapers, Earth is saved.
If your EMS is 2650-2799, you unlock the third (Synergy) ending.
If your EMS is 4000-4999, if you choose to destroy the Reapers and "saved" Anderson, Shepard lives.
If your EMS is 5000+, if you choose to destroy the Reapers and did not "save" Anderson, Shepard lives.
Saving Anderson means not letting the Illusive Man kill him outright.
EDIT: The game assumes if you do not import a save then you DESTROYED the Collector's Base
If you saved the Collector's Base in Mass Effect 2, then:
If your Readiness Rating (War Assets multiplied by Galactic Readiness, will be called EMS from here on out) is below 1750, Earth is destroyed regardless of your final choice.
If your EMS is 1751-2049, if you choose to destroy the Reapers, Earth is also destroyed.
If your EMS is 2050-2349, if you choose to control the Reapers, Earth is saved.
If your EMS is 2350- 2649, if you choose to destroy the Reapers, Earth is devastated but still saved.
If your EMS is 2650-2799, you unlock the third (Synergy) ending.
If your EMS is 4000-4999, if you choose to destroy the Reapers and "saved" Anderson, Shepard lives.
If your EMS is 5000+, if you choose to destroy the Reapers and did not "save" Anderson, Shepard lives.
Saving Anderson means not letting the Illusive Man kill him outright.
_____
If you did not save the Collector's Base in Mass Effect 2:
If your Readiness Rating (War Assets multiplied by Galactic Readiness, will be called EMS from here on out) is below 1750, Earth is destroyed regardless of your final choice.
If your EMS is 1751-1899, if you choose to control the Reapers, Earth is also destroyed.
If your EMS is 1900-2349, if you choose to control the Reapers, Earth is saved but devastated.
If your EMS is 2350- 2649, if you choose to control the Reapers, Earth is saved.
If your EMS is 2650-2799, you unlock the third (Synergy) ending.
If your EMS is 4000-4999, if you choose to destroy the Reapers and "saved" Anderson, Shepard lives.
If your EMS is 5000+, if you choose to destroy the Reapers and did not "save" Anderson, Shepard lives.
Saving Anderson means not letting the Illusive Man kill him outright.