USP is made in either PG or VG - not a mix of the two. Pure nicotine is extracted in the USA and put in either USP PG or VG to be sold, at the concentrations mentioned earlier, up to 100mg/ml. Therefore, one might purchase a liter of 36mg/ml Nic in PG (or the same in VG, but not a mix of PG and VG).
If one wanted to make a 50/50 PG/VG mix of 12mg/ml from 36mg/ml Nic that is in PG, then this presents a mixology situation. For example, if the 36mg PG Nic was mixed 50/50 with the same amount of USP VG (remember that USP has no Nic, whether it is PG or VG), that would make an 18mg 50/50 mix. But to make 12mg in a 50/50 mix, one would have to add some USP PG as well, to cut the 36mg PG Nic.
100ml of 12mg/ml 50/50 PG/VG, with no flavoring added...
First, the 36mg/ml PG Nic needs to be cut to 24mg/ml (we'll make 50ml)
X33.333 ml of 36mg PG Nic
+16.667 ml of USP PG
X50.000 ml of 24mg PG Nic
X50.000 ml of 24mg PG Nic
+50.000 ml of USP VG
100.000 ml of 12 mg Nic 50/50
One could do the same with 36mg/ml VG Nic cut with VG USP, and then add PG USP.