This is a frustrating 'lack of feature' - but there are a few workarounds. here's a quick and dirty option - but generally acceptable: At the bottom left of the composition window, just to the right of the magification popup, is an incon / popup - select 'Title / Action safe'. This has crosshairs to mark the centre of the composition. If you then twirl down your timeline and select your 'Mask Path', and then hit Cmd or Ctrl T on the keyboard you'll get a bounding box (a transform box) around your mask. This includes a visual centre point. You can then zoom in and manually align that centre point to the title/action safe crosshairs. Double click inside the mask to exit the transform mode. it's crude - but at maximum magnification it should do. A second option: If the layer you are masking has a source (ie: it appears in the Project panel), you can use this tip: Add your layer to the Comp and have it selected. Then, rather than drawing the mask shape, simply double click the appropriate mask shape in the toolbox (eg: double click on the rectangle). This will give you a mask shape that exactly fits your layer - and is centred on that layer. You can then select the mask and hit Cmd / Ctrl T to bring up the transform box. Start to drag a corner handle and then hold the Ctrl key - this will scale the mask shape around it's centre point. Double click inside the mask to exit the transform mode.
... View more