I don't understand the difference yet. Please, tell me more about it!
Maybe you can see some difference in the examples below. In translation 就正好 will not always be rendered as two words.
Fantastic! That's just right.
The station is straight ahead, this road leads right by it.
If you look beyond the trees, then (就) you can just (正好) see the village in the distance.
In Western countries it was customary to fill mattresses with hay, thus the meaning then (就) (of hit the hay) was just (正好）to say "time for bed ".
On the contrary, if if the fire is starting to go out and you place a log on the smouldering embers, well the log will then (就) just （正好） put out the fire.