Freakyuser,
It might not be your hamstring muscle. If you can touch the floor with your knuckles without bending the knees, then it is most likely not your hamstring. Also, this is beyond average (most people can't do this).
You might want to check and see if your lumbar spine is the problem of your stiffness. However, you can read the Functional Movement Screen and see if you can test your own hamstring flexibility.
You can watch this video on the active straight leg raise and see it for yourself.
When you said, "I feel a stone-hard hamstring." First of all, this is pretty normal because that's when you start to put your sciatic nerve on a stretch, so your neuromuscular response is to stop your muscle from going any further. This is done to prevent injury.
However, you might want to stretch your piriformis muscle, since the sciatic nerve runs straight throught it. Give your piriformis muscle a stretch and see.
So the answer is that it is pretty normal for you to feel this way at that stretch point. However, you might want to start working on your lumbar spine mobility and stretch your piriformis. Check out the lumbar spine mobility exercises and see if you can start working on your lumbar spine mobility.
You can also try dynamic stretching since it's been proving to be more effective overall.
Good luck!
