|In the Agile method you are better finishing 1 story of your sprint 100% than be 90% finished with 5 agile stories.|
If you don't know about Agile, it's a technology method to get things done and iterate.
You break your work into 2 week cycles called sprints. Within this sprint you pick stories (something that has value to a user) to work on in this time frame from a backlog of stories, doing the most important ones first.
At the end of the 2 weeks you demo what you've built and get feedback from your users. Using this feedback you adjust and
I've made my career at doing 90% of the work then moving on.
Success means to finish something.
Find something that's easy to finish and get to 100%.
Since nothing is really "finished" you could also timebox your task or size it (1 page, 100 words) etc.
It's not really finished until you post it somewhere and get feedback.