API 620, Roundness of the Storage Tanks

Hi dudes,

Can anyone help me out with the specifications of API 620, Roundness explaining:

"The skirts or cylindrical ends of formed tops or bot-toms shall be sufficiently true to round so that the difference
between the maximum and minimum diameters shall not
exceed [B]1% of the nominal diameter[/B]"

Currently we are constructing Storage Tanks with 74 meter diameter. so, considering the specifications of the API 620, how can we measure its roundness? I'm a little bit confused now!:upset:

your helps are greatly appreciated.