I used what is probably the simplest setup, which is a highfield lever mounted on the mast to tension the wire:
I used an old wire halyard to measure the length of the wire needed, but positioned the highfield after the wire had been made just to take account of small errors and get the usual tension somewhere in the middle of its range. The only problem was to properly tension the genoa & mast while I measured the length – I put a the setup under proper tension with a temporary block and tackle onto the spinnaker ring on the mast before measuring.
Other tensioning options include muscle boxes or block arrangements usually somewhere near the centerboard case. You may be able to get good tension more easily and/or adjust on the fly. Others are better able to advise on these things.