Digital watermarking is a key technique for protecting intellectual property of digital media. Similar to the noise pollution, watermarking process may induce image degradation. Since image quality is a determinant of its worth, it is significant to adjust the parameters in watermarking algorithm to satisfy specified quality requirement. Based on the Laplacian distribution model of AC DCT coefficients, we deduce theoretical relationship between the scaling parameter in a typical watermarking algorithms and signal to noise ratio (SNR) or peak signal to noise ratio (PSNR) of watermarked images. Thus the scaling parameter can be estimated according to the assigned image degradation grade. Experimental results demonstrate that the estimation has enough accuracy.