Coefficient DCT

Dear all,

Some watermarking technique is done in DCT domain.
To embed the watermark bit we can modify DCT coefficient,
for example I use the embedding formula : C'(i,j)=C(i,j)+alpha * w(i,j)
but,there is problem in decide factor alpha
How we can set the alpha factor rightly?
There is a fix rule for it?


Thank you,
Reinhart