Adolf,
I'm using the OS120 w/pump and mine required about 1/4 oz in the tail. I'm using a Ram glow driver and it's up all the way to the firewall. My total weight is just over 12 lbs.

I think this is just the nature of the beast. I flew mine the first time and got it down ok, but on the second flight I flipped it on landing just as others have.

Good Luck!

Steve