If you want some calculative value in a field using some other fields of the app, then below is how your basic flow should look like: 

In this calculation use case we are calculating the working hours (Duration) of an employee based on his In and Out time. The Working hours are automatically calculated when "In Time" and "Out Time" is entered in the app item.

Step 1: Event 

Click on "+Add Step", select "Event" and then "+Add".

Select the target application, event type (the flow initiates as per the event selected e.g. if "When item is updated" is selected, the flow will initiate when item in the selected application's item is updated) and the filter conditions (optional).

Step 2: Calculation

Click on "+Add Step", select "Calculation" and then click on "+Add".

In the calculation window, Select the fields for calculation from left (Fields from the selected application in step 1 lists here) and write the code for what calculation you want to perform on these fields.

Step 3: Update

Click on "+Add Step" ,select "Update Item" and then click on "+Add".

Select the field that you want to be updated after calculation. In this use case we are calculating the work duration of the employee using the "In Time" and "Out Time" of the employee.

Step 4: Workflow Type

Select the workflow type for when you want the calculated value to be added in the item.

Step 5: Save, Test and Publish

Save the workflow after all configuration by clicking on the Save button ,Test it and Publish the workflow.