Nic shot calculator
Tell it what's in your hand and what you want to end up with. It does the maths.
Why the numbers look odd
Mixing isn't linear because the shots add their own volume to the bottle. Adding 20ml of 18mg shot to 100ml of 0mg gives you 120ml at 3mg, not 100ml at 3.6mg. The calculator handles all that, but if you ever want to check by hand: shot ml = (target × base ml) ÷ (shot strength − target).