Both the tube type and the amplifier topology (Class A or A/B). To keep it simple: it's maximizing power, minimizing distortion and maximizing tube life. From there the bias is a function of the plate voltage and impedance of the OPT primary.
When I built my KT150 amplifier, I needed an OPT wound for 6K at 3 ohms and 65 Watts since my speakers have a flat 2.5-ohm impedance. That determined Class A/B operation. Selecting a plate voltage of 460V I set the bias at the recommended 70% (110mA) and turned it down from there. Sounded great until about 95mA and then turned it down to the final setting of 85mA to prolong the life of those $150 tubes. That's what I would recommend if I was selling.
Maybe Ralph from Atmashpere can comment... knows more about this than me.