Trial and error? It depends on your trainer, your ftp, and possibly a few other variables. My direct-drive (wheel off) Kickr 2017 setting for an FTP test is around 35% with an FTP of around 220-250.
If you happen to have the same trainer, for something like the Kolie Moore test I would use standard/level mode (more like outside vs resistance) and set it to level=2.