I mix my own. For simple math, I buy 60mg nic in VG. I mix 10% nic base, 10% DW (yes the gallon for a dollar at the grocery store), 80% VG. That is 10ml nic base, 10ml DW, 80ml VG for 100ml bottle.
Now I dropped to 3mg so 10ml nic base, 20ml DW and 170ml VG for a 200ml batch. Simple and Cheap...