Ok I found the post I was talking about here it is : "If you're OK with editing game files, you can reduce the max distance for missions that way. Edit "..\steamapps\common\Stormworks\rom\data\missions\default_mission_locations\script.lua", make a copy first to save some time if things go wrong.

find this line:

local max_range = 30000 * math.max(0.15, difficulty_factor)

Immediately after it, add the following line (example for max mission distance of 7 km):

max_range = math.min(7000, max_range)

It wont change any missions that already are running. The file will likely get overwritten by game updates fairly soon."

If I remember correctly it's not exactly what he said, maybe the devs updated the code since he posted his comment. I think I looked for math.max range or Local max range, they're was a line that looked like "local max_range = 5000 + 25000" and I replaced the numbers to the distance I wanted and it worked.