How to easily determine the distance between points in 3D space.

“My character is 50 feet away from the base of a 60 foot cliff and firing my shortbow (range 80/320). How far away is the target standing on top of the cliff? Do I have to roll at disadvantage or is it still short range?”
“I have a fly speed of 60 feet. My opponent is flying 45 feet away (as measured on the battlemat) and is also 25 feet higher than me. Can I fly to within 5 foot of him on my turn?”

I have been using a simple technique to quickly determine the distance at my table. First I determine the horizontal distance and the vertical distance. Take 1/2 the shorter of the two (rounded down) and add it to the longer. When all of these distances are in 5 foot increments and the distances are…