Use the boosts BEFORE 55. They get multiplied by the 12x bonus. After you hit 55 you'll only be getting ~50k xp from quests, so you'll only get 12.5k bonus. The best time to use them is on Makeb, where you'll be getting 500-600k per quest, but you'll have way more than enough to last all of Makeb so feel free to start using them anytime 1-50.
What I usually try to do is play without the bonus most of the time, bank up 3 class quest rewards in a row, then pop a boost and get all 3 xp plus bonuses at once. From that point the boost will last long enough that I get another boost token before the first ran out.
I've also used the 'banking quests' trick to jump straight from 55-57 before I even start the Revan content. If you don't know what the trick is, basically when you complete a quest you can just hit X on it, then at any time you want, wherever you want, you can click it open in the top right corner and claim your rewards. The thing is, the XP you get is based on the level you completed the quest, not the level you claim the reward. So while you're on Makeb and are getting close to 55, just start banking quest rewards instead of instantly claiming them (you can only bank up to 3; after that it won't let you start new quests). You'll hit 55 from mob or exploration XP, then you can claim those quests, and they will still have the 12x bonus! If you have the right quests banked, and make sure to have a 25% bonus token active (plus 30% legacy, and 10% from guild if you can) that gives over 2 full levels of XP instantly.