Although I agree that using a blank disc is technically wrong, I have found to my experience that it's actually quite useful. Firstly, ALL bias settings are a compromise, as it depends on the modulation level, the frequency of that modulation, and the radius from the centre of the record (and the stylus profile). Generally, using a blank disc results in underbiasing compared with the correct method LTig outlines above. However, I suggest that underbiasing is actually a Good Thing, as it protects from groove jumping when playing less than pristine records.
With a scratch or a piece of dirt, the stylus will be lifted relative to the bottom of the groove, and with even correct bias, the cartridge may skip to the previous groove, to be repeated again next revolution. With underbiasing, such lifting will still encourage the arm to swing inwards and there's a much better chance of just getting a click, and the record continues to play. I have therefore deliberately underbiased my turntables, as most of my LPs are purchased used, and although clean, are not always damage-free.
I use a blank lacquer as my blank disc, and don't have a problem with mistracking, but I accept I could probably get a couple of dBs better tracking with the correct setting at the price of skipping on my poorer LPs.
S.
