It depends on weighted average of USD value of job.
Like if you give your client 50% rating in first job of $100 and 100% rating in second job of $250. The overall weighted average will be 85.71%
The contract amount doesn't figure in. A higher-paying contract won't have more impact on JSS than a lower-paying contract. It's the total, with other factors such as a significant number of inactive, open contracts; private feedback; repeat clients; etc., etc.
ETA: Sorry, "it's the total" sounded odd there in re-reading...here.