Is it 20% or 25%?

Google Shopping Europe retains up to 20% of the CPCs.

That means a €1 CPC bid is effectively reduced to 80 cents. To counteract this, I would need to increase my CPCs by 25%. My CPC would then be €1.25 and if 20% is subtracted from that the effective CPC is €1.

Since using a CSS neutralizes the negative effect of GSE, the effect is equivalent to up to a 25% increase in CPCs.