Not saying any way is better, just wanted to re-state your formula in a different way for so as to drive home the math.
I always pick my target volume and target nic content first. Knowing I want 30 ml of final product at 20% and have 100 mg/ml nic liquid I multiply 20% (0.20) times 30 ml, giving 6 ml 100 mg nic juice needed.
I then add the other ingredients to the 6 ml nic liquid until I have 30 ml total.
If I want the same 30 ml, but at 12%, I multiply 30 ml times 12% (0.12), telling me I need to start with 3.6 ml nic liquid, and add other ingredients until I reach 30 ml final product.
This is why I really like the 100 mg/ml nic liquid, makes for easy math. It gets more complicated with 60 mg/ml nic liquid, you can't use your target mg/ml final product percent directly.