If I were you I would consider preliminary the minimum nominal thickness from EN mentioned adding corrosion allowance. EN gives you some numbers in between of those calculated by you. Be aware that there are differences when tank has welded saddles or loose-fitting type. I would use a rough hand calculation or PVElite just to confirm wind and seismic scenarios/ saddles load and tank thickness.
Some things do not need to be deeply calculated, that's the real engineering.