Just add a bit of PG and VG - and your nic level will be reduced.
You do lose a bit of flavour that way, but if you are vaping on a good setup, as you are - there is not much of a drop in flavour - at least the drop is not a problem at all.
Obviously the best way of reducing nic is to have a zero mg version of that flavour - then you just add that and dont lose any flavour.
The calculations are quite easy
If you have 18mg juice and you want to halve it to 9mg, you just add half of the 18mg and half of your PG/VG mix.
If you want 12mg, just add two thirds of the 18mg juice and one third of the PG/VG mix.
I generally use a 50/50 PG/VG mix but if I want a bit more clouds, I make it a bit more VG.
I'd also recommend trying it on a small amount of juice first. You don't want to ruin an entire bottle if you overdo it and completely kill the flavour.