[TIMTAPE]

Yes that was a pretty standard method. A more precise method used the "overbias" level of say a 10kHz tone. Correct bias for a given tape might be when increasing bias increased 10 kHz to its maximum and then decreased it by maybe 3 or 4db.

More pro tape decks had the extra flexibility of a high EQ (pre emphasis) gain trimmer, one for each tape speed, plus adjustable bias for each tape speed. So you could optimise bias and high frequency response somewhat independently. The OP's Thorn seems limited to adjusting only the bias current. Going further seems to involve swapping components.

Bias level was always a trade off between good high and low frequency response consistent with low distortion. The discovery of AC bias in the early 1940's was a crucial moment in the development of high fidelity magnetic tape recording. Before that, magnetic recording wasnt taken very seriously as its distortions were far worse than the current disc recordings.