Tune the old fashioned way.
Assume that someone knew something as a start. This may be wrong but makes a useful hypothesis. So, get the masthead centered athwartships by hand tensioning the uppers and slack lowers. I find the main halyard bounced side to side is a good measure. Run the main up in calm air and look at what, if any, mast rake it takes to get the boom level. Tension the backstay and head stay to keep that. Get the lowers hand tight to remove any mast bend. Now go sailing. Assuming that your boat trim is right - water is a bit denser than air so get this right before fooling with the rig - get rid of any lee helm or excess weather helm. Get the head and back stays tight enough that you loose any excess cup in the jib's luff. Make sure that the uppers are tight enough that the masthead does not sag off and the lowers are tight enough that the mast stays straight tack to tack. It makes little difference whether you tighten on the weather or lee side as you tack back and forth so long as you tighten way less than half what seems needed with each evolution. You should end up with the leeward shrouds just barely unloaded on each tack.
This gets the mast in colum. From here, depending on use and sail, you can go on to bend the mast for flattening the sail, etc.
G'luck
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