Powder bushing charts are just a general guideline to get you approximately where you want to be. Just for that reason, a scale is necessary.
As well, when you first start with newly added powder, and at points there after, its best you go through a few powder drops (not actually loading those shells) and then start weighing. It takes a bit for the drops to level off and get consistent. Once you're comfortable the selected bushing is dropping how much powder you want and is relatively consistent, then you only need to weigh every 25 loads or so just to check.
Other factors like static, temp, and humidity will affect drops. For instance, when you first add the powder to the bottle, you may find you got consistent drops around say 18gr. Then after sitting idle in your basement for two weeks, you go back and its now dropping around 17.
