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Actually, I mentioned the gravity drive twice in my first paragraph in my first post. The book actually mentions both weight and mass in the same sentence leaving it vague what it actually means. And for roleplaying purposes your average person isn't going to do physics calculations let's be honest. 

We don't know how finely calibrated it is but then again, we don't need to. That's up to each GM to decide.

In all honestly, a fraction of a degree deviance from the intended course over those kind of distances will result in massive course corrections. The idea is actually mentioned in the very first mass effect game at the very start, when Nilhus congratulates Joker on only managing to miss the intended target by several hundred km. Now in Alien, where space travel isn't facilitated by relics from ancient technological civilizations, this is going to be more of a problem.