Mobile Calculator UI Design

Daily UI Challenge
Day 4.
One area of design that I've struggled with for a while is that of UI design. More of my background in art is focused on fine art, such as drawing, painting, photography, etc. However, the thought process, logic, and reasoning behind UI design are extremely interesting to me and also exciting. As in anything in life, practicing something that we're not good at makes us better at it, and that's my goal with this challenge - to become a better UI designer by the end of the hundredth day.
Any input or advice would be greatly appreciated.