the only trick to this method is your nic content
if you make your premix 6mg
then when you add flavor it will be lower. depending on how much flavor you add. s if you use FA flavors which are super concentrated and add 5% flavor your 6mg nic is now 5.7mg if you use cap or tfa and end up adding 20% flavor your nic is now 4.8
you can try to compensate for that by making your base a little stronger in nic. but how much? depends if all your flavor mixes are the exact same percentage then you can figure that out. but if one flavor is 8% and another is 17% and another is 5% you can't figure it out.
but I have made up my pg/vg/nic ahead of time, LOts of people do. Some even make huge batches of the flavors in a premix, their pg/vg/nic in a premix, then just mix the 2 together when they need to.