[Inkscape-user] Granularity along inch rulers.

I am working on a book about covers using the Perfect Cover Generator. This works in inches. I need to tell how to draw some additional guides at distances of 0.25 inch from the generated guides marking the spine. However the rulers along the sides only measure in inches with 1/4 inch tick marks. I need at a minimum 1/8 inch marks to approximate the correct positioning.

How can I refine the rulers to show these 1/8 inch subdivisions? I can draw my own ruler along a path or convert temporarily to pixels and count off 18 pixels but either of these techniques seem a bit clumsy.