You are correct. OG is the number you use to calculate ABV. There are online calculators you can use. I'm terrible at hand written notes myself so I committed to a program long ago. BeerSmith is my choice. Makes it easy to keep all the info right there in one spot, just plug in your raw data and it does the calculations for you.
Also as a bag brewer you'll find that even with a full volume mash you'll likely get a little higher efficiency than expected. I BIAB with more traditional volumes and a sparge. My mash efficiency is close to 100% on beers below 1.080 so I calculate all my recipes on 85% except bigger beers which I do at 80. I just anticipate it and adjust my grist bill slightly. I'm still consistently over what the software predicts but that consistency is the key. I know what to expect and plan accordingly. That's the only reason efficiency numbers matter. Higher efficiency isn't some measure of brewing prowess, it's just a measure of your system's ability to extract and process sweet wort from grain.