H! I've build this pedal and it is my first Rangemaster alike npn booster. It sounds as it should I think. The pot Range not only boosts the highs at higher settings, it also boosts the lows at minimum.
Most of the diagram is based on Ohm's Law: V = IR (voltage = current * resistance).
If you need help figuring out how to get from Ohm's Law to the equations in the diagram, let me know. Memorizing them works, too. I just find them easier to remember when I know how to derive them.