your best bet would be to buy the same juice (or similar flavor/different vendor) in a higher nic then add the two together in equal parts
so if you are trying to shoot for a 12mg and you already have a 3mg buy another 10ml bottle of 24mg and mix the two together
basically if equal parts it would be 3 + 24 = 27/2 = 13.5 (that's about as close as you are going to get)
unless you want to figure out the math involved in doing 1/3 and 2/3 parts
