Here is how I get super long (but not high) flights with power armor jetpack: It's been a while since I played FO4, so going from memory for now. 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). The total ground you can cover, however, depends on thrust management. (At approx 20 meters using very rough vertical measurement by setting POV in fixed position and using reference features on buildings, etc.)
Per all my testing the height appears to be capped. The other answers supplied were very helpful, but didn't state the best solution which, in the words of Charlie Sheen, isĪgility points help flight and action points help duration.
