it seems that in the past Guinness records there are no standards other than sling and have an official measure the range. I don't even know if they measured bounce. and they probably used a wheel to measure.
in the current record the thrower used a dart and a sling that can't sling stones and replaced the older record done with stone instead of making a new category
Wind is important and should definitely be taken into account but it is not easy to measure. I'd suggest at least recording the direction of the wind at slinger's position.
from our contemporary slingers you can watch videos on Irongoober's channel he has done some 300+m throws.
from older slingers you can see the records
https://www.slinging.org/ranges.htmlLarry Bray 437m 52g stone 130cm sling
below a picture with the stone and sling and a lead projectile that would go further. picture is from CA or Larry Bray I don't remember who posted it.
check out also the records from Vernon Morton and Melvin Gaylor they slung some heavyweight stuff with probably shorter slings
If anyone can film a video like IG I'd like to see it, even finding a place to sling that far is difficult.