Stumped on setting a warning when LineTotal (CuryLineTotal) is less than 50

I’m new to Acumatica and trying to get my head around adding what seems like a simple customisation.  I've been trying to get a warning to show when the line total is less than 50 on a Sales Order.  I have looked at other examples of how to raise a warning and tried various things but I’m not entirely sure what event handler it should go in as it’s not a user updatable field. I also think I should be using a different method to show the error but not sure what.

The following is what I have:

protected void SOOrder_CuryLineTotal_FieldVerifying(PXCache cache, PXFieldVerifyingEventArgs e, PXFieldVerifying InvokeBaseHandler)
    {
      if(InvokeBaseHandler != null)
        InvokeBaseHandler(cache, e);
      var row = (SOOrder)e.Row;

      if (row == null) return;

      if (row.CuryLineTotal < 50)
      {
        throw new PXSetPropertyException("The order total is below the Wholesale minimum of $50.", PXErrorLevel.Warning);
      }

    }

The error gets thrown as a message box on the screen before I even get to open the order that has a lower value for the Line Total. I just want it to show a warning exclamation mark next to the Line Total if it’s less than 50 once the Sales Order is displayed.

Sorry if this is a noob question.