I am trying to calculate PRS scores for several individuals for which I have several variants genotyped. I have chosen a PRS score with 40 variants, but in my dataset only 20 of those variants are present. What is the correct way of assessing the decreased predictive power of the PRS score due to the missing variants?

My thinking is that having all variants "I have a predictive power of 100%", when I miss half of them its not as simple as a decrease by half, because each of the variants have a different weighting.

Do I sum the effective weights for all 40 variants, and then compare this to the summed effective weights of the missing variants and from this get an estimate of the decreased predictive power?

Thanks a lot and sorry for newbie question!