Three tools within the overlay editor will help you position your elements within an overlay.
Rulers - toggle on rulers that will show in pixels the height and width of the canvas area.
Grid - toggle on a grid for the overlay. When enabled, the size of the grid can be adjusted within the settings dropdown.
Snap-to-grid - when enabled, dragging a panel will snap to the nearest grid line. This takes into account the current grid size and will snap to all four sides.
You can also align elements within your overlay - either to the canvas itself, or to other elements.
Align to canvas - panels can be aligned to the canvas area. When a single panel is selected, the alignment tools will appear on the top right
Align to other elements - When multiple panels are selected at once, the panels will align relative to each other.