Couldn't distance be "known" at the time mission is generated and just no shown? I mean actually that the distance at which POI is generated is depending on some (hidden) mission difficulty as well as the reward. But I have no statistics anyway to prove any of my guesses.
Nope. It's generated dynamically[1] when you are in system.
I put up this thread a while back which had a fair bit of chatter... I can't recall if we
directly addressed massacres in it, but I also recall having done it before as it's a straightforward calculation which prompted me to make that thread, comes out to something like 500k per kill in solo pirate massacres and 200k per kill in wing pirate massacres[2]. Civilian massacres are a bit harder because they pay better, which means they often hit the 50m reward cap, skewing calculations somewhat... as a fairly rare mission it's just more effort to actually get some usable stats.
I'm not saying you
couldn't pre-determine the distance at mission-generation time... the game simply doesn't do it is all, and it may not be possible for the game to do it in it's current implementation. I can imagine the information required to do so would bloat generation time for these missions substantially, compared to others.
[1] You can work this out by "forcing" the Mission USS to spawn elsewhere, which is done by forcing an instance change... in binary systems this might be over 100kls difference... given that you could notionally "force" USS to always be close to you. Regardless, while it's definitely no "smoking gun",
which would be confirmation of the actual code or it's functions, it's close enough to be an accurate measure.
[2] It's how you can tell CZ massacres are broken... the math for them is back-to-front, with wing massacres paying more per kill than solo missions, while all other solo massacre variants pay more per kill.
EDIT: I should probably add, by looking at missions which
do offer range-based bonuses, you can get a feel for what they look like in those missions, and contrast against ones like massacres, salvage and assassinations... that makes it pretty apparent that these don't get any range-based bonuses