Formula Summing field working based off a Checkbox field.

Need help creating a formula. I have a Formula Field that calculates (sums) the Invoice Amount (Numeric) with the Retainage Percentage (Percentage/ Relationship Field) and provides the Amount of Retainage that needs to be withheld on said invoice. I would like this field to only Sum though based on a Checkbox Field being Checked. If the Checkbox isn't checked then the field doesn't calculate.