Display message based on value entered in text field

I feel like this is a simple task, for someone that knows what they are doing.  I have a text field in LiveCycle that the user is entering a numerical value in.  When the field is exited, I would like a message to be displayed after calculating the value entered.  For example, the message will be displayed only when a value of 40 or greater is entered.   I assume that basically it should be something along the lines of IF field.rawValue > 40, THEN  display message "you must enter justification" 

Can someone provide me with the exact syntax that should be used?  Thanks for your assistance.