The best scented candle and melts come from using quality ingredients. Following proven measurements, temperature and tested over time. A good quality candle will have cold scent throw and good hot scent throw.
A cold scent throw is the amount of scent produced when the candle is unlit, a hot scent throw is the amount of scent the candle produces when lit. There is a fine line between the two. It takes science and practice to get the amount of fragrance right so you get good hot and cold throw from a candle. There are many candles out there that smell great before burning, but nothing when lit or vice versa.
Each type of wax has a threshold amount of fragrance oil you can put into it. A certain % per pound you can add, it also depends largely on the temperature you add the fragrance to the wax as well. If your fragrance has a flash point lower than the temperature you add it to the
wax, the fragrance oil will burn off before the candle cures. The wax manufacturer has a specific pour temperature for the best adhesion to the container and smooth finish, so fragrance needs to be added above the pour temperature. If you are using large wicks, or multiple wicks depending on the type, they can burn hotter than the fragrance flash point, and will burn the fragrance up to quickly so the hot scent throw will not be as good as it should be or none at all. This can also affect the burn time of the candle. You want to get the best burn time, so that the candle doesn't burn up to quickly. You need the right temperature burn from your wicks to not burn too fast or hot so you get the longest burn possible with the most scent throw.
Aromatherapy Candles or essential oil candles are even more difficult to get the hot & cold throw perfect. Essential oils have a large difference in flash points, so they need to be researched to find similar flash points with the base, middle and top notes that compliment each other for the purpose intended for Aromatherapy. This is a daunting task sometimes.