This is actually a great myth in our industry. I say that because it's not an exact science by any means or accounts. The problem here's that I know for a fact that you guys have been led to believe that we can follow a 75 by as little as 3.5 miles as long as we're above his glide.
We're not asking for you to fly 3.5 miles behind them, but if you could do around 4.5 or so on a visual it'd help out a lot. If you're going to stay at 5 miles or more...meh, I might as well just give my usual IFR clearance, which again goes to show why some of us are reluctant to give visuals sometimes.