I've got some help for you, even though I don't know the gap length of that head.

There is an alternative method of setting the bias that doesn't depend on the gap length. Just use 1KHz, and overbias by a quarter of a dB. Or two tenths of a dB if you're really picky. This works with any tape, either 7.5 or 15 IPS, and any head gap.

This is just as accurate as using 10KHz when you know the gap length, and much more accurate than using 10KHz when you don't know the gap length.

You can see a nice discussion of this in the MRL publication "Choosing and Using MRL Calibration Tapes" paragraph 7.1.