All e-liquids are made up from 4 basic ingredients.
1/ Pure Vegetable Glycerine. (known as VG)
2/ pure Monopropylene Glycol. (known as PG)
3/ Monopropylene Glyco flavouring
4/ nicotine. (usually a mix of pure nicotine mixed with 80% Vegetable Glycerine and 20% Monopropylene Glycol.
So lets try and keep this simple. When you go and by a 60ml short fill at say 80/20, it contains a total of 50ml of liquid. 80% VG and 20%PG and the PG part is carrying the flavouring. If you add a 10ml nic shot which contains 18mg of nicotine you get a 60ml bottle which now has a nicotine content of 3mg. (The 10ml bottle of nicotine which contains 18mg of nicotine is diluted down by the 50 millilitres of liquid already in the bottle and now by volume only contains 3mg of nicotine.)