I always mix up my nicotine base in bulk in advance. It saves time and also reduces the potential to accidentally corrupt the precious and expensive stuff with a flavour. If you know that the average percentage of additives in a mix (flavour + sweetener + ethyl maltol) is circa 25% you can compensate. I mix a 33 mg/ml base and a 28 mg/ml - the former for sweet flavours the latter for tobaccos and it is easy to arrive at 24 mg/ml using these.