You can absolutely use mica in soap, although some testing is required to determine exactly how much mica you'll need to achieve your creative vision.
While weighing on a scale is the best way to accurately measure a portion of mica, the portion of mica that you'll typically use is so light most scales will not register its weight. However, we have a starting point to share!
Mad Micas, who creates the micas we carry, recommends using 1/2 teaspoon of mica per pound of M&P soap base, which is a great starting place. After testing, you can adjust up or down to achieve your desired shade.