I have used kitty litter too, in similar proportions to those quoted - mostly for my "growing-on" trees, rather than the "finished" objects, works very well in my opinion. One word of caution though - as well as avoiding perfumed/deodorising brands, be aware that some litters absorb moisture and retain their solid integrity, and some just turn to mush! If in doubt, before adding to your soil, put a test sample in a pot of water for half an hour and see how it behaves...