You can't prevent your soap from shrinking. Curing your soap mostly means letting water evaporate after a week or two. It's a necessary part of the process. That's why you can't store your CP soap in airtight containers while it cures.
Soap will continue to shrink for about 6 months, though most of the water you'll lose will be within the first 2 months. You should weigh some freshly made soap then check the weight again in 2 months and again at 6 months to know the exact amount so you can make a good guess on how to label your weight up front.
In my experience, you'll lose about 15% of the weight in the first month, another 5% in the second month and a bit more but not much more over the course of the rest of the 6 months.
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