Let's say the first field is called Text1 and you want to multiply its value by 8 and show the result in Text2.
You can do it using this code as the custom calculation script of Text2:
event.value = Number(this.getField("Text1").valueAsString) * 8;
Also, see this tutorial: https://acrobatusers.com/tutorials/how-to-do-not-so-simple-form-calculations
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