Keep adding some paces until it can reach the top of the launch where the model is close to being over the stake and the line is all off the ground and about 1/3 of the rubber is lifting up a little. That's about your best shot. But under no circumstances should you stretch the rubber longer than about 3 times its length if you expect a reasonable life out of it. It'll take 4 times but it'll start to show signs of wear much sooner. If at 3 times the rubber length the line is still not all off the ground then you have too much line. Shorten it and you'll actually get a higher launch. Pace out the rubber just sitting and then stretch it by twice that number of paces.

But you need to remember that you can't get higher than the line that's on the high start.