Lynda, you bring up a great point. So great, that you've actually persuaded me to buy 100 next time. The less of the nic base you have to use, the less chance of an accident. With my 75/25 PG mix, a 30 ml batch will use 12 ml of nic base. With 100, I'll only use 7.2. Not quite, but almost half as much. Yeah, the 7.2 is just as strong as the 12, but dealing with a smaller amount is easier.
If you're really worried about it, then find a way to secure the bottle. Pop the top off, and suck a few ml's out with a syringe. It's really not that scary. But if it scares you to the point of stress, stick with 60. You'll thank yourself in the end after having to deal with the weaker bases. It opens up your options for change as you see fit.
Also, it doesn't pertain to your issue, but I thought I'd mention the rule of thumb. If you like to have a higher percentage of PG, then go with the PG base. With a higher percentage of VG, then use the VG base. 50/50 doesn't work so well. I just used the calculator (accounting for 10% flavor), and with 50/50 PG needs to be as much as 30% and VG needs to be as much as 20% for the numbers to come out. I imagine that you know what you like your mix to be. If you like mostly PG in your juice, then chances that you go mostly VG one day will be slim to never.