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I have a 1KG x 0.1g increments and another scale that goes to 10Kg in 2g increments, both from EBay. Between them I can get everything weighed accurately. However, I noticed that on EBay you can find scales of 5Kg in 1g increments, and that's what I consider ideal for everything except the tiniest ingredients.
The main thing to consider is how accurate you need to be.
For instance, let's say I am making a skin cream and let's look at the major/minor ingredients. The major one is of course water. So this is going to be about 80% let's say, and I am making a 1Kg batch, so I need to weigh 800g of water. Now, what accuracy would I like? In general I consider that 1% accuracy is good. (Titrations in chemistry using a burette only get 1% accuracy) So, I need to be able to distinguish 1/100 x 800g = 8g intervals; my 10Kg scale is fine for that because it can distinguish 2g steps.
Let's say the next ingredient is shea butter at 4%.
I need to weigh 0.04 x 1000g = 40g. The small scales will allow me 0.1/40 accuracy = 0.25% no problem.
OK, now let's look at the smallest % ingredient, which is dimethicone petrolatum at 0.2%
I need to weigh 0.002 x 1000g = 2g. The small scales will allow me 0.1/2 accuracy which is 5% accuracy, not so good, but it will do, because in general I am not too worried whether I get + or - 5% too much/little of the ingredients at the smallest concentration.
Going to a scale better than 0.1g would be possible, but the problem then becomes different; it is difficult to use such sensitive scales because the surface they are placed on, and air movement, upsets the reading and it can make it very time-consuming to measure.
I hope that's demystified the scales . . . please work in metric, the old fashioned system is a nightmare and encourages serious errors. Working in metric makes relating % to actual weight a snap.
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