LFO source: Square
LFO dest: pitch
how can I set the amount to be a perfect octave?
what should be the exact number in precision mode?
I don’thave my Phatty right here but to me the best way is to listen.
The only detail is that as LFO is bidirectional (I think so at least…), the note you’ll be playing when pressing say a C key will be a F#, i.e. half an octave. If you set the LFO amount to play a two octave interval, that is not true anymore and all your problems are gone 
The LFO is not bidirectional when you use square source.
Zero problem then 
Precision mode is pretty precise. I am pretty sure it will differ slightly from phatty to phatty. Use your ear or a tuner to find the precision mode value for your particular phatty.
A good way to do it with using your ear and no tuner…
Mod wheel down, press a note.
Mod wheel up, press the same note but an octave lower.
Adjust amount knob to get pitch closer
repeat until you cant hear a difference.
Nice