Big Bud + hammerhead PK 9/18 = True love?

i have just started with BigBud, its now day 12 of flowering (bigbud by now is OK right??) and i was wondering if i should start adding hammerhead PK 9/18 as well, ive heard that BigBud is the same as pk 13/14 ? but 9/18? damn, i wish i knew wtf it was :S

question: Should i start adding BigBud and PK 9/18 now?

day 12 flowering, 600w hps, coco

i wouldnt bother using both mate, p=13 and k=14.... p=9 and k=18
there both the same thing but with different quantities of p and k
im not sure but the 9/18 is more suitable for hard water areas

thx alot for your input! anyone else got something?