I often use a bracket with a serious offset that no doubt produces an unbalanced light. I feel it does not matter, regardless of where the beam of flash hits the brolley it will be widened and diffused. Providing diffused light falls on your subject at the desired location the intended photograph should come out alright.

Duncan, saw your jewellery shot in the gallery.

Few things you should keep handy while shooting jewellery. White cards of various sizes. Small mirrors of various sizes. Small support and Blue Tac packet.

Gold/silver needs lot of reflectors to enhance their tone and shine.

For jewellery, use white acrylic sheet (if not soft box) instead of umbrella. Light concentration will be much stronger.