Not sure what you are referencing, but the space loot system is just a bell curve. Each named part has a mean and a standard deviation hard coded, eg Quantum Ion Drives have a Top Speed mean of 92, with a standard deviation of 4.6. This means that a QID with a top speed of 101.2 is 2 standard deviations away from the mean, with a 2.5% chance of occurring. Going out to 3 standard deviations puts you at 105.8, with a 0.15% chance of occurring or 1 in 667. Where things start to get near their limits of realism is 4 standard deviations, 110.6 top speed, or 0.006% chance of occurring aka 1 in 15,700.
By setting 50% to the mean, and 100% to 4 standard deviations, you would end up with a pretty accurate reflection of the rarity of parts. Where this becomes tricky, and what I assume you are referencing NSG brought up, was that the different named parts have different means and SDs. The way pilots have categorized this for as long as we've known about it is basing the rarity on the named parts with the best drop chance for that stat. When you talk about the rarity of 8 engine Top speeds, you dont factor in the non QIDs because they don't have a chance to drop good top speeds, and the market naturally corrects to top speeds being more valuable because QIDs are a quest reward rather than standard loot. Where it becomes subjective is deciding what rarity top speed to pair with say a 95% rarity mass. There's never going to be a perfect answer to that because its going to come down to how many QIDs are being generated on your server. Despite that, the tables are not dynamic and do not shift over time. 95th % is 95th % forever