It depends on what your target nic level is. W/ a 6mg base once you add flavors that percentage is going to lower ie. if you add 10% flavoring to your nic base then the resulting liquid will be 5.4mg.
As long as you have a few supplies, are safe, and use an 'e-liquid calculator' then you can safely and accurately use any level of nic base to achieve any desired final percentage/ratio.
What I do is buy 100mg/ml nic base, some 60ml bottles (to prepare my mix in), a few 30ml bottles (for daily carry), 2 luer-lock syringes, and a few bulnt-tip luer-lock needles. I use one needle and syringe to measure and put in my main mixing bottle. Then I use another syringe and needle to draw, measure, and dispense my pure PG and VG (both USP grade and purchased on Amazon). After that simply add your flavorings.
I use this great online calculator to get all of the measurements right, no guessing, no equations to mess with. Have a look, it works very well and you can save your recipes and add notes to it so you can tweak and improve your recipe as you gain experience.
e-Liquid Calculator :: New recipe
-Hope that helps, and Best of Luck-