I see that Planck scale of mass-energy is related to the corresponding length scale by application of the uncertainty principle. What is not clear for me is how we find either energy or length scale to compute the other one? What goes wrong if we assume a higher energy level than the Planck energy scale and then find the corresponding lower length scale via the uncertainty principle. Is it related to the gravitational constant? Should we use the concept of a tiny black hole from general relativity? Also, how we compute the Planck time for given energy and length scales?