I spent much of the last waxing moon making my first batch ever of magical chalk. Today I tried out the finished product. Clearly my skills in this still need some work. The chalk is too hard - I can use it on concrete, and probably asphalt, but it fails on wood, plastic, and cardboard.
I will try new proportions of ingredients, vary the ingredients some (corn starch should work the same in magic as white flour, shouldn't it?), etc.
A witch's work is never done. Just like tweaking recipes for food, recipes for magic take practice and I don't know if I'll ever call them 'perfect'.