I recommend a different procedure when mounting the Heavyweight on your tonearm. The Rega RB300/600/900 arms use a spring to adjust the stylus tracking force, and several companies that modify the Rega arms recommended that you disengage the spring.
This is done by dialing the tracking force dial up to the maximum force (3.0 grams), which disengages the spring that applies tracking force (I know this sounds counter-intuitive, but the spring actually lifts the arm, rather than applying downward force). After you have disengaged the tracking force spring, use a stylus force gauge (such as the Shure SF-2, which costs $20 from The Audio Advisor) to set the tracking force by sliding the Counterweight forward or backward, and then lock the set screw that secures the Counterweight to the tonearm stub.
I think you will be very pleasantly surprised by the improvement offered by the Counterweight. When I mounted a Counterweight on my Rega RB-900 about 2 years ago, I was startled at the overall improvement to the arm's performance.
This is done by dialing the tracking force dial up to the maximum force (3.0 grams), which disengages the spring that applies tracking force (I know this sounds counter-intuitive, but the spring actually lifts the arm, rather than applying downward force). After you have disengaged the tracking force spring, use a stylus force gauge (such as the Shure SF-2, which costs $20 from The Audio Advisor) to set the tracking force by sliding the Counterweight forward or backward, and then lock the set screw that secures the Counterweight to the tonearm stub.
I think you will be very pleasantly surprised by the improvement offered by the Counterweight. When I mounted a Counterweight on my Rega RB-900 about 2 years ago, I was startled at the overall improvement to the arm's performance.