Whats the relation between AC power strip current rating and DC output current from the PSU connected on
It's enough to just calculate the power consumption in Watts on the PSU (Vout * Output current) being lower than the one rated for the power strip?


Yes. Power requirement of the PSU is \$ P = VI = 12 \cdot 20 \ = 240 \ \mathrm {W} \$.

On 230 V mains the current required by the PSU is \$ I = \frac {P}{V} = \frac {240}{230} = 1 \ \mathrm {A} \$ (plus a little).

Your power strip is fine.