if your 100ml bottle is full it has 300mgs of nic. you want to bring the sum to 900mg/100ml. naturally by adding liquid your total ml's will change, but still...
as the ppl in the Shiniytis thread answered:
if you add 6mls of your 100mg/ml nic base that means you'll add 600mg nic and have a total of 900mg nic / 106mls of total liquid. which comes to roughly 8.5mg/ml nic.
if you add 6.5ml of your nic base you'll be closer: you'll be adding 650mg to make a total of 950mg / 106.5ml of total juice. which makes: 8.9201877934272300469483568075117 mg/ml.
hope your bottle has more space in it or you have a larger one.
if not you could take some out and vape it: take out 6mls of the original juice and add 6mls of the 100mg nic instead. you'll have a total of 8.82mg/ml
this is all freebase nic? i know many can DL it, but that'd hurt my throat. mtl i can do higher but not much with freebase nic, i couldn't do 12mg for instance.
gooluck