You've got an easy situation there because you want your nicotine at exactly half the concentration it is now. So if you mix equal amounts of your 48mg/mL liquid and the PG, you'll get 24mg/mL in a 75/25 mix.
You're probably going to add around 10% flavor (VERY ballpark guess). That means your flavor would dilute 24mg/mL to around 21mg/mL. You could add 20% flavor and still have over 18mg/mL of nicotine.
So... if you truly need to have your final mix be 24mg/mL, bust out that calculator. If, on the other hand, you aren't too picky and are willing to have liquid that's somewhere between 18mg/mL and 24mg/mL, you can save a lot of effort by just mixing the PG and nicotine base 50/50, then measuring out how much unflavored liquid to flavor you want.
The complicated part is when the mix MUST end up at a certain concentration. If I was making a large batch I'd care about this. For personal use the latter way is much easier. Besides, I'd be willing to bet (real) money that someone couldn't reliably discern a 10% nicotine concentration difference blindfolded.