you need to keep I think its 1/3 of the plant mass left after harvest. switch it to 24/0 light schedule and start pounding nitrogen into her. If you google it you can find some great guides. The trick is when to regen. you don't want to wait until the very bitter end of flower. you want to do it while there is still some life left in her. clip it like your taking clones.
After a few regens. quality and potency begins to decrease. Alsothe more regens the less likely it will be to survive. May I ask why you are regening it?
mesohi....harvest the top 2/3 of the plant or so. Leave as many of the lower leaves as you can. Put it back ubder 24/0 with a dose of "Vegging" ferts, and hope for the best. Some strains regenerate pretty easily, others won't.
Though I have done this in the past in order to preserve a specific strain or phenotype, regenerating to reflower, is a total waste of time and effort..IMHO.