I just started DIY e liquid over the last two weeks. There are some great calculators that can be found throughout this site, as well as plenty of recipe sites that have calculators that help tremendously. I started out trying to make 6mg juice, but I've since stepped down to 3mg instead as I've found it easier to measure out the smaller amount while still allowing a little wiggle room for adding a little too much nic. My first few batches, I added a little too much nic and it was not a good experience.
I'm not going to lecture you on safety (well maybe I am a little), but if you put "a little too much nic" then you weren't doing it right.
I'd tell anyone doing DIY that they should know exactly how much nicotine they are adding to their mixes - there is no wiggle room - use a syringe to measure it accurately and double check your calculations before and after adding it.
Getting it wrong could result in a lot more than a 'not good' experience - particularly so if you are using higher nicotine strengths.