EN 14015 vs API 650 reinf. thk calc difference

[MariusLT]

Hello all,
Question regarding flush-type clean-out doors.
I need to calculate bottom reinforcement plate thickness.
Opening dims 800x300 mm. Tank height 6 m.
API650 3.7.7.1, table 3-11 states 1/2 inch (12,7 mm).
EN14015:2004 Annex O gives formula 7*sqrt(H+3)=21 mm.

Why such a difference?
Formula error in EN, maybe cubic root?

Thanks in advance.