Approve Job Offer - Need help with creating rules using offer grade rates and offer salary amount

Summary:

Approve Job Offer approval workflow is failing when using expression builder to derive the Grade mid point.

Content (required):

Trying to implement the below business requirement for Approve Job Offer approval workflow"

1. Check for the Offer salary amount against the Grade Max, min or mid-point of the proposed grade to which the candidate is being hired.
2. There is no Grade mid-point attribute available in the Approve Job Offer approvals.
3. Derive Grade mid-point using BPM expression builder (in advance mode) as :

Offer Salary Details.Salary Amount > = ((Grade Min + Grade Max) / 2)

The above rule is not working and the workflow is failing. Checked for null values for offer salary amount, proposed Grade min and max values, still workflow is failing.