Project project = CreateTestProjectWithCustomField();

// Set Formula
ExtendedAttributeDefinition attr = project.getExtendedAttributes().get(0);
attr.setAlias("Task number fields");
attr.setFormula("(([Outline Level] + [Priority] + [% Complete])/2");

Task task = project.getRootTask().getChildren().getById(1);

// Print extended attribute value before and after updating task percent complete
System.out.println(task.getExtendedAttributes().get(0).getNumericValue());

task.set(Tsk.PERCENT_COMPLETE, 50);
System.out.println(task.getExtendedAttributes().get(0).getNumericValue());