Lazy old frat here.
I mix my base at slightly over my preferred nic % and slightly over my preferred VG %. (For me, that's 20 mg/ml in 60PG/40VG.) I get my nic in PG, and most flavorings are in PG as well.
When I mix, I use an appropriately sized graduated cylinder to mix in, and 3 or 5 ml needle-less syringes for measuring out the flavors. Into the cylinder. Add my base to the proper level, mix well, decant into bottles and wait
P.S. That's the base at 20. Final mix down is about 18 mg/ml, and closer to 65/35 on the PG/VG, depending on total flavor %. )