When to stop drying is fairly easy...I go through the math of calculating moisture content in our newsletter article:
This is the best method to determine the actual moisture content. The problem is that you need to basically destroy one of your samples. That's fine if you have hundreds of pounds, but the homegrower may not want to do that.
For the home grower, I would just assume you start at 80% moisture content and calculate a dry weight.
So, the formula is:
M% = ((Ww ‐ Wd)/ Ww) x 100
M% = moisture content (%)
Ww = wet weight of the sample
Wd = weight of the sample after drying
Normally, we get the wet weight and dry a sample out completely to get the dry weight. What I'm suggesting for the homegrower is assume 80% and calculate a dry weight...
Assuming a wet weight of 10 oz:
Wd = (1 - (M%/100)) x Ww) = (1 - (80%/100) x 10 oz = 2 oz
Now assuming you want to get to a final moisture content of 10%...
Wfinal = Wd /(1‐ (M%/100)) = 2 oz/(1-(10/100)) = 2.22 oz
I should note, for commecial processing, we need to get down to 8% to 12%. If you are using them whole cone, you should be below 20% at least. The lower the better from the standpoint of longevity.
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Or the even simpler solution is to see that the final weight is 2.22 oz/10 oz = .22 = 22% or ~1/4 to 1/5th the original wet weight. So weigh your initial sample and when it weighs 1/4 to 1/5th the original weight, you should be in the right ball park.