It has been years since I used a Tobias, so I hope I remember correctly.

The two "internal" adjustments are in the back if I remember correctly, these are to adjust "zero" and "drift" from a calibrated step wedge.
The zero is obvious, the drift is to make sure you have measurements that agree with the "curve". For example if you have a calibrated step wedge and you read step number 5 at 0.65 density units and your meter reads 0.69 then you adjust the "drift" to make it read 0.65.

The "zero" in the front is used when you want to baseline your readings. IOW you want to read the transmission only with out the b+f.

Since this is "uncalibrated" and they did not sell it to you with the standards, the simplest thing is to take Don on his offer to "calibrate" a step wedge for you. Buy a Stouffers uncalibrated step wedge, send it to Don, have him read the densities and write them on the outside of the packaging envelope and send it back to you. You now have a "calibrated" step wedge that you can use to calibrate your densitometer.