do it by feel!! that's how I do it, I think for most grower they do their whole crop all at once and it can be more stressful but I have always ever grown perpetual where I harvest a plant here and a plant there, I feel over the years I have really gotten the hang of it. If I have to go to work and it's almost ready, I might bag them into 2 paper bags, then a plastic bag, the extra paper absorbs more moisture. Sometimes Ill de-stem all the flowers, but give em another 12 hours off of the stems before bagging or jarring. The main thing is keep a few buds with a little more of a branch on them, once it has been in the jar for 24 hours, try to snap it, if it does, keep trying every now and again. I have had weed that looked so dry only to find a week later it was stored with too much moisture. Different time of the year where humidity is different and everything changes. You have to go by feel, don't be scared, and dont be in a hurry to bag it. That said Im not greedy if I over dry a plant it does not cost me a lot of money and it really doesnt happen that often anymore. You want the weed to feel soft but the stems MUST snap.