here is a link for the calculator that I use.
eJuice Me Up - Best eJuice Calculator Install and then just fill in the blanks. I have given you the percentages for the Smooth Menthol just add in that you want to make 6ml and the mg per ml of nic. IN order to make a final eliquid with a strength of 12 mg you would need at least a 19mg per ml of nicotine base or you would need to reduce the % of flavoring. If you want to reduce the % of flavor by 1%(so instead of 17% it would be roughly 16%) plug in these percentages:
Wintergreen 2.82%
Menthol 2.82%
Koolada 2.82%
Spearmint 4.7%
Sweetener
2.82%
= 15.98% total flavor
which means you are going to add 4ml of nicotine (18mg strength) and 1.04 ml of either (or a combination) PG or VG, your preference, to come out with 6ml of final juice of 12mg per ml.
No mater what PG/VG ratio you use the final nicotine strength will be 12mg per ml.
I like to use this analogy when trying to explain using a higher strength nicotine base in DIY. Picture this ...a glass of iced tea...now think of your flavors as the ice cubes and the tea is the nicotine strength. The more ice you add the weaker the tea becomes unless you brew stronger tea to begin with to compensate for the dilution by the ice cubes. So pretty much the more flavor(ice) I want to add the stronger the tea(nicotine base) needs to be to be able to come out with a given strength of tea.
Post #3....1-2 yr old is ok if it has been stored properly. Is it pretty clear? Some discoloration is expected due to oxidation. Does it have an offensive smell or taste? Try a small 3ml mix to test it out first and if it vapes ok then use it.