I have had a quick look at your file. So notes:
1) your maths used to calculate the vertical height is wrong. You are using Pythagoras' theorem but you are only calculating allowing for a slope in one plane, you have to allow for a slope in two planes, so if viewed from above you have to use Pythagoras' theorem to find the hypotenuse of a right triangle coplanar with the top surface, then use that value as one side of the right triangle to calculate the vertical height from your angled edge length
2) I very much doubt that anyone would want to specify the planter box by the length of the angled edge, if it really is a planter box, they would want to specify the vertical height.
3) You now have a base and top that are rectangular rather than square, that is just an observation, but it will effect the maths for Pythagoras' theorem.
4) if you lined up a sketch vertical through the diagonal corners, you could use the sketch with construction geometry and constrains and a "driven constraint" to automatically calculate the correct vertical height, if really needed, if the difference between base and top lengths is the same as the difference between base and top lengths.