I’ll take a stab at this. It’s been years since I built my Super 70’s, but I believe the Gain pot must to turned full CW (to set max resistance). That way it is combined with TR1 to set the max resistance seen by the Emmiter of Q3. That should permit the correct bias range for the collector to be 4.5V to 6V.
Barry may know better. I see that the build doc does not specify this.