With the release of Xanathar's Guide, you can technically work out the rate at which you're falling. But it's important to remember that a Dungeon Master's ruling is final. Ultimately, they decide whether you live or die by allowing you to cast spells or use items during your fall.
Don't panic, though! Most Dungeon Masters have a vested interest in seeing parties complete their campaigns. So, your character will likely live to see another day regardless of how impactful the fall was.
If you're a stickler for the rules, you can use Xanathar's Guide to calculate the rate you're falling. Your character will fall up to 500 feet per round, which lasts for 6 seconds. You can work out your falling speed by dividing the time by the distance of the drop.
So the maths is very simple:
500 feet / 6 seconds = 83 feet per second
Still, no matter which rules you use, the falling damage cap will only let you take 120 points worth of damage.