Is there a ratio? I just cut a watermelon into sections, then cut a section into slices each day and laid them out over 1/2 the FT. Apart from adding some horse manure occasionally I haven't added any bedding for ages. Maybe I should!

I usually figure 1:1 by volume using the WAG method to determine the volume. Since watermelon rinds release a LOT of water into the bin, I may double the cardboard just to soak up the water.