I would never add straight nic to your tank. It should always be added in a bottle with VG/PG.
If you are trying to increase the nic level... you would add the amount of nic needed to make that size bottle in the mg nic you are trying to achieve. You'll need a liquid combining calculator for this one...
https://ecigvape.com/e-liquid-combining-calculator
For example your nic is 100mg/100%PG, you have a 100ml bottle of 3mg/70vg, and would like it to be 9mg...
you'll have to play with the ml amount of nic until the overall amount you want is reached. In this case it would take 6.7mls of 100mg nic to turn your 3mg bottle into a 9.09mg bottle.
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If you are just mixing two bottles of same juice that have different nic levels... for example
6mg + 12mg You add the 2 amounts then divide in half... this mix would = 9mg of nic.