Desired strength is your goal, meaning whatever you want it to be at the end. That’s what you want too vape, not what stands on the nic bottle!
Nicotine strength is the number that stands on your bottle. In your case 48, put that in the box.
Pg content of nicotine is the base of your nic, since you said yours is pg based set it to 100.
Since its pg based and not includes any VG, set VG content of nic. to 0%
Do not click the ready to vape base, leave it as is and save to default. Next time you click create, these numbers will be applied automatic, and you won’t have to worry, unless you make changes.
From there you can add flavors on the bottom, hope it helps.