A "doubler" will have your 30ml container loaded only half method, allowing you to fill the rest with the nicotine mix of your deciding on. If you are topping up, after that you would typically be using a PG base with a high degree of pure nicotine such as 100mg/ml. On a global scale, this base is also not so usual.
https://ziondjrj138.mozello.com/blog/params/post/3106614/fluid-barn