how many yards do you tend to loose with a range ball or is it a myth

No myth. Unless your range has very good balls, the distance is compromised. More noticeable on the longer clubs.

Rumour has it, when they set up our range, they moved all the boards nearer to take this into account.

Anything over a 7 iron, I take no notice whatsoever. Why I take a mental note (in about 0.02 seconds) of the distance with short clubs I don't know. Force of habit I guess.

I know if I hit the 100 board with my wedge, that'll easily be 100+ on the course, so that's OK. Same with the 150 marker, which is actually a pole in the ground, I hit that sometimes too.

As for the "nets" at 50/80 yards, I can't think of a bigger waste of balls. These tiddlers are for the practice ground.