My basic kit, for started was PG with 54 mgs nicotine, VG, PG (without nicotine), the flavoring I started with which happened to be my two basic vaped a lot flavors and citric acid, and the obligatory utensils, tools and bottles. Pre-diluted nicotine can leave a residue unless you use really hot soapy water to remove it when done 'mixing' so to avoid any risk to the family I always had separate utensils for my e-juice.
When I started with DYI I did not mix my own flavor combinations to come up with other flavors. That
's because I went to DYI once I started having to order 5 bottle of my favorites at a time to keep in stock - DYI was a cost saver. After the comfort level set in then I started buying flavors that I combined to come up with my own flavor combinations.
As for the citric acid, cause people ask, its not only a flavoring in used in certain amounts but a small amount can help bring out other flavors as well as extend the shelf life a bit. I don't like the idea of some of the other stabilizers and 'preservatives' that are often use like polysorbate 80 but since citric acid is used for flavorings it seems 'relatively' harmless to add it a a 'flavoring' to everything and prolong shelf life especially if you are mixing a larger batch.
As for you calculations as to what you finally end up with as to nicotine content, you need to get the right 'calculations' for the ingredients you use. Some of the charts are based on people that use pure nicotine because they want no PG or started in the time where PG/Nic base was not available or are just pursuits. In your case you want a chart that will tell you what you need to add to cut down the 54mg to your desired level -12 or 18 or whatever. Be careful never to confuse your charts if you switch from using a nicotine already diluted in PG or if you mix your own PG/nic base or don't use PG at all and make a VG/nic base. I have always had to use nicotine and mix my own own VG/nicotine base if I want high percents of VG then PG or wanted a PG-less e-juice.