What additional tools do i need to start stringing?

Thanks for pointing out the major flaw in the calculations. it gave me a chance to check it again. Infact full distance from gripper end to the center is 11cm. I earlier simply divided by 2 to calculate at 5.5. This time I realized that that gripper actually starts at 3cm from the center and rest is an even central block. That means the mass center of the gripper would be at 7cm rather than 5.5 cm. So, I just recalculated again here are the results gain. For me a design issue should not be discarded just by a vague comment on the practical scenarios. here it is not just about consistency, which can be attributed to the law of averages, but also about if you put 10kg then do you get consistently 10 kg or you get consistently 9.5kg. In theory on average it would be 5% less in case of 10kg and 4% in case of 12kg.

Yes, there maybe all kind of variables on real life scenario when someone drives a car on the road. But, it does not give an excuse to leave 10% variation of the roundness of the tires or breaking distance of a car.

So, if anyone finds it is ok to continue by ignoring a design issue, it is fine. one the other hand , if someone wants to look deeper and adapt as @jnsjnsn did here, it fine as well.