For a parallelogram with sides $a$ and $b$ and area $A$, you could use the measure $\dfrac{4A}{(a+b)^2}$. That will be $1$ if the parallelogram is a...
Sorry, my mistake. I should have written 3.39.38.13 there (I have now corrected it). The idea is that if you want just one of the three cards to be a...