Nic shots are used with shortfill products (nic shot ready / overboosted flavor).
Shortfill products contain 40ml mixture in 60ml bottle, so you need to add 2x 10ml nic shot.
If you add 2x 20mg nic shot at the end you will get 6.6mg mix
If you add 1x 10mg and 1x 20mg nic shot, at the end you will get 5mg mix
And if you add 2x 10mg nic shot, you will get 3.3mg mix