The most simplistic and straight-forward interpretation, is measured perpendicular to the Dip Direction, so converting simply requires -90, but then a correction factor applied to any negatives to subtract from 360.
To do this in Mapinfo -
1. Create a new Column for the Dip direction. Then use Table Update Column and minus 90 form the strike
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2. Now select from this Dip Direction table, the rows where Dip Direction has gone below 0, using Table>SQL Select -
3. Now update the selection query, to subtract the values from 360 -