You can calculate the cost to complete in two ways. You can calculate the cost to complete based on forecast transactions, or you calculate the cost to complete based on totals.
Based on forecast transactions
The forecast transactions specify the cost to complete. Total costs are calculated by adding the forecast transactions to the actual transactions:

Actual + cost to complete = total
Based on totals
You enter the total costs as a lump sum. The cost to complete is calculated by subtracting the actual from the total:

Total  actual = cost to complete.
Topics about cost to complete
Topic 
Description 

Conceptual information about the different ways to cost to complete. 

Conceptual information of cost to complete; include examples. 

How to apply the cost to complete. 