diff --git a/lib/editor/canvas/pen_editor_screen.dart b/lib/editor/canvas/pen_editor_screen.dart
index c48e87b..b09a882 100644
--- a/lib/editor/canvas/pen_editor_screen.dart
+++ b/lib/editor/canvas/pen_editor_screen.dart
@@ -15,6 +15,7 @@ import '../engine/undo_stack.dart';
 import '../input/diagnostic_logger.dart';
 import '../input/pen_config.dart';
 import '../input/pen_input_service.dart';
+import '../layout/viewport_fit.dart';
 import '../persistence/editor_repository.dart';
 import '../persistence/save_scheduler.dart';
 import '../ui/pen_settings_page.dart';
@@ -381,11 +382,12 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
     showPenSettingsSheet(context, config);
   }
 
-  /// Centre [pageSize] within [viewport] via the shared transform.
+  /// Centre [pageSize] within [viewport] via the shared transform. Uses the
+  /// shared, unit-tested [centerOffset] (pageSize is already fit to the viewport
+  /// at scale 1, so we center at scale 1).
   void _centerPage(Size viewport, Size pageSize) {
-    final tx = (viewport.width - pageSize.width) / 2;
-    final ty = (viewport.height - pageSize.height) / 2;
-    _transform.value = Matrix4.identity()..setTranslationRaw(tx, ty, 0);
+    final o = centerOffset(pageSize, viewport, 1.0);
+    _transform.value = Matrix4.identity()..setTranslationRaw(o.dx, o.dy, 0);
   }
 
   @override