When you say "headings", the correct term in Microsoft Project is summary tasks. When you describe the "heading" that sits over all the tasks, the correct term is Project Summary Task, which goes by the nicknames Row 0 or Task 0. So, make sure that your first task in the project has the ID number 0.

Display the Custom Fields dialog, select your custom field, and then click the Graphical Indicators button at the bottom of the dialog. In the Graphical Indicators dialog, select the Summary Rows option and then select the "Summary rows inherit criteria from nonsummary rows" checkbox. Then select the select the Project Summary option and select the "Project summary inherits criteria from summary rows" checkbox. Click the OK button.

In the Custom Fields dialog, also remember to select the Use Formula option in the middle of the dialog. Click the OK button and this should resolve your problem.