The idea of biasing the tape is this:
When you record on a tape, the bias oscillator in the deck generates a signal of around 100KHz, at 100V peak to peak, to erase the tape. This is the erase current, and it goes to the erase head. To stir up the magnetic particles on the tape, to make it easier to record on it, a smaller portion of the erase signal is fed to the record head, or the record / play head on a 2 head deck. This is the bias signal. If the bias signal is too high, it will saturate the tape, (I think it actually is self erasing the recorded signal) and you will lose highs when you record. If the bias signal is too low, the highs will be accentuated and you will get higher distortion. The method of adjusting the bias is to record a 1KHz tone and a 10KHz tone at -20dB (so you will not saturate the tape and lose highs) and adjust the bias so that they are equal on playback. You can then sweep the signal up in frequency and see where it rolls off by -3dB. For the best high frequency extension, you can adjust the record equalization so a 15KHz tone at -20dB plays back at the same level as the 1KHz tone. If your deck doesn't have adjustable record eq, and you want to get a bit more high end extension, you can adjust the bias so that the 10KHz tone is +1dB from the 1KHz tone, but no higher.
After adjusting the bias, you should adjust the internal record gain so that a 1KHz tone recorded at 0dB plays back at 0dB.
Of course, the first thing you should have done to the deck is adjust the playback level so an alignment tape with a 0dB 400 Hz tone plays back at the 0dB level for the deck, and adjust the meters for 0dB.