To create a custom field on Customer records to show the target sales per Customer:
1. Navigate to Setup > Customization > Entity Fields > New
2. Provide the Label
3. Set the Type to Currency & Store Value = enable.
4. On Applies To > select the Customer
5. On Display tab > select Subtab to Main
6. Click Save & Apply to Forms
This field can now be populated with the target amount of sales coming from this Customer.
This information now can be sourced into reports. If you run a report like Sales by Customer (Navigate to Reports > Sales > Sales by Customer) pull the custom field under Customer/Project folder. By doing this you can compare the Actual Sales vs. the Target Sales per Customer.