2600-3000 hammer units is a good rule of thumb for acceptable sniper sightlines. There are only 2 sightlines longer than 3000 in any official TF2 map, at least that i'm aware of. The start of badwater basin is one of them, from the back of the BLU setup gate room to the back of the hill where snipers peer over, which works out at about 3100-3300. It's relatively balanced with the fact it's a secondary path and that the more distance you give yourself the narrower your FOV and thus lowered capacity to shoot multiple enemies. You can check the details yourself. The other is on upward in an area that sees little combat, just before CP3, and i think works out at 4000 units at max. The sightline isn't easily exploited; players are too exposed and the sightline sees only a minority of activity.