You certainly can adjust acidity after fermentation is complete, but many winemakers feel that the acid is better-integrated, as well as less-detectable, the earlier it is added. That being said, by all means measure the total acidity after fermentation (and after malolactic fermentation) and feel free to adjust as you’d like. Indeed, adding tartaric acid will shift the pH down (more acidic). Looking at your wine’s numbers above, I’m guessing that post-malolactic fermentation you ended up with a pH in the range of 3.75–3.85, depending on the amount of malic acid in the grapes. Your grapes don’t strike me as being particularly ripe, so be sure you do bench trials on your finished wine in order to check that you’re not over-adding. The great thing about home winemaking is that sometimes the best thing to do (adjust based on taste) is the easiest as well. Too much added tartaric acid can make your wine taste harsh, sharp, and tart. Acid also has the tendency to emphasize tannins; if your wine was green or unripe tasting in any way be careful not to add too much.
If you have access to a pH meter you can do step-wise bench trials to see how a specific measured tartaric acid addition affects your wine. My gut feel is that you may like a 0.25-0.30 g/L addition of tartaric acid but be sure to add in increments to make sure you like the results. Just be sure you use tartaric acid. Malic acid, of course, will just get consumed by malolactic bacteria and will disappear, whereas citric acid can actually be used as a food source by some wine organisms so its use as an acidifier is always risky.
