Tinkering with things to make them better can sometimes ruin the entire project. Even moving farther away from the goal, sometimes... Knowing what "done" will look like, and accepting it, is an art form. You can always make a version 2.0 if you feel there is more to add or modify, but recognizing what "done" will be when you start will help you recognize it when you get there.
I write this today because over the years there were a lot of projects I should have determined what they'd look like 'done' when I was getting started. It probably would have helped me keep from altering my original idea as I made progress. It would have also kept me from 'combining ideas' on things that should have been two separate projects.
Perhaps the most challenging part of Entrepreneurship is knowing when you are done with something--and when it is time to begin something new. :)