The other answers supplied were very helpful, but didn't state the best solution which, in the words of Charlie Sheen, is
Agility points help flight and action points help duration.
Per all my testing the height appears to be capped. (At approx 20 meters; using very rough vertical measurement by setting POV in fixed position and using reference features on buildings, etc.)
The total ground you can cover, however, depends on thrust management.
I was able to fly from the big island in bottom right corner of map (the one you can occupy, forgot the name) to the shore (near the old refinery/dung farm which looks like shortest island-mainland distance). It's been a while since I played FO4, so going from memory for now..
Here is how I get super long (but not high) flights with power armor jetpack:
- Max natural agility (perks + bobblehead)
- Agility paintjob on armor
- Food items that grant bonuses to AGI and AP (deathclaw steak, mole rat chunks, cooked softshell meat, and maybe a couple others, I think.)
- Drugs that grant bonuses to AGI and AP (usually carry x100 jetfuel and hotkey it.. constantly addicted to Jet :D )
- Maxed Perks that increase power armor efficiency and movement speed (Though I have not yet confirmed that this has any effect; just stating that I had these before even crafting the jetpack; need to test with different unbuffed char next)
- Hotkey Ultrajet (although Jet works pretty good too since you can quickly regain the "high" with hotkeys)
- Make a running jump, and use Jet as you are running or jumping.
- Focus on distance, not height. The trick is to manage thrust carefully, tapping jump key only as needed, NOT holding it down. This is sorta possible without jet, but much easier while time is slowed. If you run out of jet, take some more! (Yeah, you'll get addicted, but don't worry! Addiction is a joke in this game compared to the previous ones; 4 diff ways to cure it. Or get perk where you never get addicted to chems.)
Also note that
Jetpack + Quad-barrel Missile Launcher + Jet = Archangel of Death