has anybody used humbolt gravity?

i'm get n ready to start wk 6 today of flowering and i'm gonna try that gravity to harden the flowers. i understand the amount to use of it............. but what i don't get is when it says use a 1/3 of my nuts in the res with it. wel my point here is some guys runn ther ppm up to 1500 and some much higher. so what is to high of a ppm to use with the gravity? thanks for anybodys input......i already see some of my yield being slim do to my mistakes along the way. so i'm just trying to make the best of what i got. my next batch is gonna be ten times fastr and better. i'll post some pics in a few wks.
 
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