Adjusting bias on 2 head machines

[Victory Pete]

I recently set the bias on my TSR-8. I am using RMGI 911. It is suppose to be bias compatable with 456 which is what the machine is designed for. The manual states to set the bias voltage to 35mv. I am sure this is close enough, the RMGI actually sounds better than any 456 I have used, or at least that I remember. How can I tweak the voltage to pinpoint the optimum setting? Here is the problem: I understand how to set the bias for 4 db over bias on a 3 head deck, I figured I would just have to keep increasing the voltage 1 mv at a time in 10 second increments as I let the tape record. Then I played back the tape and watched the results and made a graph. The problem is that the lowest voltage I can get the trim pot to obtain is 20 mv, as I increase from 20 to 21 to 22 and so on, the bias curve had already peaked and continued to fall through the whole experiment. I cant determine how far overbias I am at any given point on my graph. If I could get the trim pot to start at a lower voltage I would be all set. Confusing ain't it.
Victory Pete