Well that is the general idea of how it works, you grab current position (or any position that never changes, e.g. a fireplace or any object position), and then generate a path using that relative position,
e.g. in pest control, the north east portal is ALWAYS going to be for example position(+100, +50, 0) of the boat coordinate, so it just adds onto it like you said.
If that doesn't work, make sure you calculate the current position correctly, so perhaps grab the position of the npc that shouts (at the beginning of the game?) or something that is in the game, that ISNT in the normal pest control bank area.
This would've been a longer post if you provided some code or some more detail as to why it didn't work
EDIT: If you need more help, contact either @Valkyr or @Khaleesi as they both have pest control scripts and they may be willing to give you some advice.