Thanks for replies.
Actually I tried another mast extension with four 'wheels' before making these pics, trying to see if the extension matters.
As Al Planet said, my mast is constant curve and the Severne sails in SDM also constant curve (according to Unifiber mast selector), so it should be ok. However the Severne website shows 'severne mast 460 ' in their recommended mast column in the NCX specs.
Should I try to rig it a few more times or return to the seller because its unfit to my sail? (Though the seller told me it is ok to rig with NCX before I bought it) ?

Annoyed and frustrated. Everyone said how good the sail is but I feel its troubles and not yet I can enjoy its goodness.