I would suggest for you both to get ejuicemeup calculator
Its my favorite go to and free!
eJuice Me Up - Best eJuice Calculator
When using 48mg 50/50 and you want to get to 18mg 50/50 you would need to add to your bottle (you will not be able to use 48pg nic to get to your 50/50 blend)
30ml batch size
11.25ml of 48 50/50
6.375 of PG
9.375 of VG
3ml of Flavor ( this is at 10%, adjust this number to taste as it depends upon the flavor)
When using 100mg pg based to get to 24mg 50/50
30ml batch size
7.2ml 100 pg nic
4.8 ml pg
15ml vg
3ml flavor again adjust this to taste - 10% flavor in this example
Let me know if you have questions