Gallon? Just how many ml of juice do you go through per day?
One gallon is about 3785 milliliters.
If you go through about a dram (little dropper bottle) a day, then that's
almost 3 years worth.
I think most folks that "mix" their own buy unflavored nic juice a little stronger than they normally vape, some PG, some VG, and some flavorings.
Maybe something like this:
3 30ml bottles of PG based 36mg/ml tobacco flavor that you like
1 16oz bottle of VG,
a handful of 1 dram flavors, from maybe Lorann,
maybe a 16 bottle of PG,
heck, toss in a 100ml of 60mg/ml unflavored.
That would be a good start and give you a 3 month supply.
Gallon?
search for one of the "calculators" on the forum.