There is a set of measuring spoons with both 1/3, 3/8 and also 3/4 and 1.5 tsp measures in it (also 1/16, 2 Tbs, etc.). It is here: http://www.pourfectbowl.com/products.php?product_id=3
re: beating times, if the cake is falling apart tender, that suggests that more beating is needed, rather than less
re: cakes being crumbly, Rose’s priority in formulating cakes is that they be the best cake to eat, which means they are very, very tender by design. Great for eating but more difficult to handle, especially in larger sizes. It may be that you are doing everything right (sounds like it), but that you want a little more structure to ease handling. You could try increasing beating times or reducing the baking powder a little.
good luck with the wedding cake!