I decided to test the chance of a ranged unit to hit and miss a target under a tree because I wanted to confirm the information on Liquipedia. I created a simple UMS map that would gather data for me. I used a sieged tank for the attacking unit and a zergling for the unit being attacked. Here is the data I accumulated after ~11 hours:

Hits: 3417
Misses: 3005
Total Shots: 6422

Hit % = 3417 / 6422 = 0.53207723451 = 53.2077%
Miss % = 3005 / 6422 = 0.46792276549 = 46.7923%

Liquipedia has the Hit % at 53.125% which is extremely close to the value I gathered from my test. Even during Misses the zergling still received splash damage and it seems to act like dark swarm where the attack is shifted in front of the unit being attacked. I slightly modified my map to be able to test High Ground miss chance and I will gather some data when I go to sleep and post the results tomorrow morning.

I just realized I probably could have set up multiple interactions that gathered data so I could have a much higher sample size. I'm a complete newb map maker so it was hard enough for me just to get this simple interaction to work. I'll try to change this for the high ground test so I can have a much bigger sample size in shorter time.