It's important, and responsible, to know ones limit when dispatching targets and good to hear that's a consideration of yours.
You mention variables and that's part of what I love about long range shooting. I also cull on a big cattle farm, partly open hills and bushland, mainly kangaroo's and deer. There's variables there also, however the range is out to 800m should I deem it safe enough on a shot by shot basis. Mostly though, 100-300m.
There's so many small things that have to come together to shoot at distance and I find it all very interesting although it does my head in at times but there's a certain satisfaction of a decent stall and getting a little closer as you say...unless those deers have AK's.