What
@Various Reviews said - need to know what you are trying to achieve. If you have an 85/15 pre-mix and 18mg nic shots then I would do something like the following.
Heisenberg should be mixed at between 10 and 20% - so try a small batch at about 15% and see how you get on with it and then alter it from there. The flavouring is all PG and the nic could be either - it might say on the bottle but you said it doesn't matter what ratio you end up with so not too much of an issue.
Easiest way to mix 18mg to 3mg is to think that you need to dilute it in a ratio of 5 non-nic containing juice to 1 part nic at 18mg - you then end up with 6 lots of the quantity so 18 divided by 6 is three. So for ease of use a 10ml 18mg nic shot needs to be mixed with 50ml of non-nic liquid including your flavouring.
15% of 60ml is 9ml - so you need 9ml of Heisenberg. Whack in 41ml of your base mix and then stick in your nic shot. I can't be arsed to work out what the final PG / VG ratio as it is making my head hurt
Long-term get PG and VG separately and get some scales and beakers - mix by weight and then you can chop and change PG / VG ratio as you like. Different ratios will work better in different set ups and the PG / VG ratio will also affect the flavour too.
But as a rough idea go with the above, 9ml concentrate, 10ml nic shot, 41ml base mix. Whack that in a 60ml bottle and give it a shake and you are good to go!
*Just saw you clarified the nic is VG but that doesn't make too much difference.