REFILLING: How to determine PG/VG & Nic. Milligramage amounts, Amount a cartridge holds?

[PGVG]

How can one determine how much PG/VG juice to mix with how much of a given nicotine milligramage to add to ONE cartride?

IE:
15ml bottle of 36mg nicotine
15ml bottle of some mix of PG/VG or just PG or just VG

Or, does it come as 1, 15ml bottle with some mix of PG/VG AND 36mg of nicotine?

And how much does ONE cartridge hold, .1ml, .025ml... (I know cartridges vary by manuf. and size, but any idea?)

HELP?

 

[Sparrots]

I buy both VG based juices in 10-30 ml bottles and 36 mg of nicotine and then cut it with PG at 50-50 ratio's to get 18 mg and still have a good flavor.