Once I found my recipe I loved, I multiplied it out to large batches, so I only make juice every other month or so.
I use the DIY flavor concentrates from smoke for pennys because they have flavor and sweetener pre mixed.
I use 100mg/ml nic concentrate, it makes the math easier.
1. With a batch size of 250ml, and a 10mg/ml juice I need 25ml of nic concentrate. I measure this with a syringe.
2. I know that my flavor concentrate needs to be 2.5% of my total batch, so.... .025 x 250 = 6.25ml
3. I eyeball the pg/vg. This only controls the viscosity. As long as the total batch size is right, the flavor and nic concentrate will be good. So, I fill the bottle about 70% full of pg and give it a good shake. I've found that my flavor dissolves faster in pg, so I want to get it mixed in well before I add the vg.
4. Fill the bottle with vg. Shake well every hour for the first couple hours. Shake well and test it after about 24 hours.
Easy Breezy.
If you have a difficult task, give it to a lazy man, he will find an easier way of doing it.