I want to subtract a process variable from a specific field and get the result

I am a beginner with Appian. In the interface, I input values for each field of a record type and save them to process variables. In the process model, I want to reference the same record, subtract a specific field from the corresponding process variable, and perform the following actions:

If the result is 0 or below, I want to send an email without changing the value.
If the result is 1 or above, I want to save the calculated result in a process variable and send an email.
Can you guide me on how to achieve this in Appian?