can we validate calculated field value, if calculated field value is less that 100, then a message should display to user. or is it possible to add rule on calculated field value.
As far as I'm aware SharePoint calculated fields are calculated when the list item is saved, this would mean that Nintex form cannot validate against a SharePoint calculated field.
However, instead of using a SharePoint calculated field you could use a standard field which is set via a Nintex calculated variable. This would allow you to validate the field in real-time through Nintex Forms.
This video should show you the basics of using a calculated variable and associating it with a list column: How to use the calcluated value control in Nintex Forms for SharePoint - YouTube
I have done same thing, I want to validate Grand Total, if Grand Total is less than 100, then I want to show a validation message to user.
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