Order only the highest mg you are comfortable with using. If that is 100mg(which is what most of us here do) then be sure to take the necessary cautions, ie. using gloves, possibly goggles, mix near a sink, etc. If you buy some that is higher than you are comfortably using then you will be nervous and possibly make mistakes.(not good)Nic may get more expensive and only be offered in lower mg in the future. That is my fear, limiting strength of nic you can order.
@mikepetro has graciously made a simple calculator so that we can easily figure how much nic we need to purchase to last for xx years. Hopefully he will be along shortly with a link.( I will also hunt for it) It is really easy to use. Just figure what amount of nic you now consume each day when vaping and plug in those numbers. ie. If you vape 2 5 ml tanks of juice a day, then use the mg in that to figure
....3mg x 10ml(2 tanks) = 30mg daily.