Maybe a bit late on this one, butSeaturtle sorry mate your conversion science is mixed up, the gfj (especially white) kills the liver enzymes responsible for the vonversion of codeine
so taking it prior to the poppyST seems unnecessary, or just useless as mention in an above post, the morphines blood plasma levels may be hightened and duration increased though. But the codeine and other metabolites would pose a greater catalyst for the potentiation of the morphine, other naturally occuring opiates
, or whatever you preferable opiates within the tea are(hard to tell with such a mix).
It would be most logical to wait a good few hours for the tea to hit its peak (again hard to tell because of slow comeup and long duration) and then introduce to GFJ if you find ypu need it at all, as PST lasts 6+ hours I hardly see point in extending the duration unless you start on an early morning and want to go till the late evening, and with no responsible activities inbetween!
Looks like about 200-300ml Gfj should be plenty if your desire is mostly required to increase strength of the tea from little product or get most out of it, because I for one am usually still affected by PST next morning even without the juice.
Hope any of this helped, know your PST dosage FIRST! Of same batch, then add gfj if needed.