Here’s a bit of clarification:
Your ending(s) are determined by your “Effective Military Strength” (let’s call it EMS for now) bar. Focus on that bar – that is your indicator of how well you will do in the end-game.
You can maximize your EMS just by collecting War Assets in the single-player game. There is a certain threshold of these you would need to exceed (I can’t get too specific) but I can tell you there are MORE than the required amount that can be gathered in the single-player campaign.
“Galactic Readiness” is a modifier you can improve by playing multiplayer. That is to say, if you play a lot of multiplayer, you will need less War Assets from single-player to fill up your EMS bar (ie it will balance out the requirements to account for you playing in both modes). Single-player game play does not impact that bar.
EMS = success. EMS can be maximized via collecting war assets alone, even if your Galactic Readiness is 50%.”
So essentially, even if you completely ignore the multiplayer aspect of Mass Effect 3 (though I’m not sure why you’d do that. As mentioned in my review, the multiplayer is quite brilliant) all you would need to do is work that little bit harder in the single player game.