Thats the curse of Tufa there is no foolproof receipe. It kind of depends what you are making or adding or the amount of water can effect the piece/mix. With the leaves I believe someone has cast a thin layer of crete on the leaf and then added a tufa mixture on the back to help with the weight. My 2 cents worth.
Paws

Strength often comes with weight, weakness with light weight. Annoying, isn't it? :-)

You might try lightweight concrete instead of hypertufa and see if that suits your particular design and usage. What works for one use may not work for another.

Try a basic mix of maybe 3 parts perlite, 1 part Portland cement, and enough water to make it mold to the surface without too much slumping. Let it cure, then test if for strength for your use.

Then you could try more or less perlite, or adding acrylic or latex polymer additives to the water, or some fly ash or something else very fine to fill in any small gaps in the mix.

We are all reading here and trying various mixes as we go. We are the kings and queens of trial-and-error! With every experiment we learn more. We KNOW that some of it just isn't going to work! And some of it will turn out better than we expected.

Sue