I'm not so good with calculators, so I do everything by hand.
If you only have 15 ml of VG nic, adding 60 ml PG nic would give you 75 ml of 80/20 PG/VG nic.
To get 18 mg/ml starting with 48 mg/ml, you would have to mix 37.5 % nic base with 62.5% other stuff (PG, VG, PG flavoring).
If x = how many ml of all the other stuff you need to add (PG, VG, PG flavoring), then x/75ml = .625/.375 and x = .625 (75 ml) / .375 = 125 ml other stuff.
That would give you 200 ml final product.
If you want 50/50 PG/VG, you would need to use 100 ml of PG (including PG, PG flavring, and PG nic) and 100 ml of VG (including VG and VG nic).
It looks like your using 19% flavoring. 19% of 200 ml is 38 ml.
So, you already have 60 ml of PG nic and 38 ml of PG flavoring. Giving you 98 ml total PG. And, you already have 15 ml of VG nic.
So, you need to add 2 ml PG and 85 ml VG to get 50/50 PG/VG at 18 mg/ml.