More accurate space bar page scroll

Currently, pressing the space bar scrolls the loaded web page one viewport worth of vertical distance. The problem with this functionality is that distance is often too great. Frequently the top or bottom or many webpages are not calculated into the viewport (such as menus); this area is hidden from view. I’d request that the space-bar-to-scroll either be user-adjustable, or that it more accurately pay attention to the true viewable area.