The kitchen scaleThere is no need to keep a detailed track of your trials and errors in a notebook if you don't bother to precisely measure and weigh the ingredients you put in your candles!
For the "classics", like a pillar candle, experience and practice will learn you how to add the correct amount of each ingredient without even stopping to think about it. But until then, a small kitchen scale (digital models offer the best precision) will greatly help you succeed in making perfectly balanced candles.
For instance, you make a double-scented candle by adding an ounce (+- 28,3 grams) of fragrance oil per pound of paraffin wax. Fragrance oils must be weighted and not measured in a graduated cylinder as you would for other liquids, because depending on the selected fragrance oil, the density might differ (some FOs are "heavier" than others, a fact you can notice as you pour your candles: high denisty FOs tend to "sink" to the bottom of the mold).
A small digital kitchen scale with a 1 gram (0.05 oz) precision and an upper limit of about 11 lbs (5000 grams) is your best choice to weigh paraffin or other waxes, Vybar, stearic acid, fragrance oils and other ingredients. The "Tare" feature (where the weight of the container is substracted from the gross weight) will make your life so much easier that it would be a sin not to use it!
If you fall for the charms of a mechanical kitchen scale, be aware that its precision ranges from 0.5 to 1 oz (10 to 20 grams), perfectly fine for waxes but not the best choice for fragrance oils or additives like Vybar or UV-inhibitor... Keep in mind that the "Tare" feature is unavailable when you use a mechanical kitchen scale.
Where to buy a kitchen scale?
General stores usually have a good range of them, or find one at Amazon
Starting from $15, both digital and mechanical models.