I use Magma to calculate the L-value, yields

E:=EllipticCurve([1, -1, 1, -1, 0]);
E;
Evaluate(LSeries(E),1),RealPeriod(E),Evaluate(LSeries(E),1)/RealPeriod(E);


Elliptic Curve defined by y^2 + x*y + y = x^3 - x^2 - x over Rational Field
0.386769938387780043302394751243 3.09415950710224034641915800995
0.125000000000000000000000000000

   $#torsionsubgroup = 4, c_{17}(E)=1.$ 

But the strong BSD predicts that 

$L(E,1)/\Omega_{\infty}$= $(#Sha(E)/#tor(E)^2)*c_{17}(E)$

We will get $L(E,1)/\Omega_{\infty}=1/16$, not $1/8$.
Why does that happen? Thanks a lot.