After doing some research, I found that while there were apps like I was thinking of, most of them were:
Not Very Nice To Use
Too Little Control
and sometimes Both.
The aim of creating this app was to make a really simple way to be reminded to take breaks, so all of the benifits of taking short rests can be easily intregrated into daily computer use.
Taking Short, Frequent Breaks from the Computer can Increase Productivity, and Reduce Eye, Joint & Hand Strain