I think the key is just having a consistent method that you can use to establish meaningful training zones. We are not going to get an accurate handle on your actual TP with any real world test - the key is the first letter of that FTP acronym.
Training indoors complicates things, maybe you can add a little arbitrary extra TSS for those workouts.
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"Your beauty is an aeroplane;
so high, my heart cannot bear the strain." -A.C. Jobim, Triste