You might try splitting the difference, during your tests(ie: around 90 - 100mA). If you don’t notice an audible reduction in presentation(lost dynamics, rounded off highs, poor focus, etc), compared to the manual’s spec, leave it there. Why waste tube life?
Target bias dictated by amp or by tube?
If my amp's (Line Magnetic 508ia) manual says to adjust bias of each power tube to 120ma on the front meters, but a set of after market replacement tubes (Cossor Valve 508) state their bias should be set to 65ma, which stat should I be following when putting in the new tube?