E:D Community Goals have global rewards given to all contributors, and also personal contribution tiers that determine what level of reward you get at the end of the challenge. Below, this post shows the different possible rewards based upon the percentage of players that have contributed as much as you have.
For instance, in the above screenshot, the Colonia Connection Goal is based around trade. I found that delivering 160 tons (my entire cargo hold) put me in the top 75%. Delivering 320 put me in the top 50%. I have now delivered 640 tons and I am still in the top 50%. Other than the raw sale of the goods I'm transporting (160 tons of Titanium nets 90k), these last two deliveries haven't made me any more money in terms of the challenge reward. It might keep me from falling below the 50% mark if other players start making bigger hauls, but perhaps I wasted my time and fuel and chaff (pirates surround these Community Goal locations).
Is there a way (in-game or 3rd party tool) to see the contribution levels for each tier? Even just seeing more fine grained detail about the challenge might allow me to calculate the tiers on my own, so any source that provides active challenge progress insight would probably be helpful.