I have also been doing this for quite awhile now -- LOVE the savings! And it really does clean. My recipe called for only 1/4 cup & that amount works fine. I've also used just the slivers of leftover bar soap instead of the Fels & it works just fine. Only problem is slivers are hard to ome by as only DH uses bar soap - all others use body wash:)
Beside the savings, there's something special about the feeling you get using something you've made (even simple laundry detergent). Yes, our grandmothers would be proud!:)