How to choose the amplification factor for the required SNR in MatLab ?

I have a given noise vector with certain variance. That cannot be changed. I have an IQ-modulated signal and it needs to be amplified to get an SNR of 20dBs. How do I go about that?

k = 3/2;                    %certain variance
n = sqrt(k).*randn(1, N);   %noise vector
noise_power = var(n);

SNR = 10*log10(var(Xiq)/noise_power)  %current SNR of the IQ modulated signal

amp = 13.5;      %some amplification, I can't figure out what it should be
Xamp = amp.* Xiq;
SNR = 10*log10(var(Xamp)/noise_power)   %SNR in dB should be 20 dBs

Currently I am using a fixed scaling factor but I know this isn't the correct way to do it. Can someone help me please?