In this game, war assets and reputation add to a score that determines your choice of endings. You can play the single player, but you will find that meeting your desired score is more difficult. However, if you play the multiplayer, reaching the score becomes easier. I don't like that I have to play this game with a calculator and a notepad, and I don't like that I am hobbled as a player if I choose not to play multiplayer.
So how about you leave the calculator and notepad behind and just enjoy the story and game itself no matter what the ending is?
You can always calculate the game in subsequent replays and try other choices and approaches or simply go watch Youtube after a while to see the different endings (no matter how good a game is i rarely ever replayed a story driven game.. especially not something so big like Mass Effect that takes days or weeks of continuous play).