If you download "UDisc" for disc golfers, there is a feature that will allow you to measure how far you threw.

Or did you mean something that can calculate how far you will sling with a given speed and angle of release?   For that, I like to use this calculator. 

https://www.desmos.com/calculator/on4xzwtdwz

I suggest using 8.0 for the mass, 0.1 for the area, and 0.2 for the drag coefficient. That gives you a pretty realistic distance for a certain angle and speed of release.

If you have any landmarks that show up on Google Maps where you're slinging, you can use that to get an approximate distance.

https://support.google.com/maps/answer/1628031

Another option is to use a laser range finder.

Using Google maps on your phone, you can mark your throwing point and use the measure distance feature while walking to your stone landing.