Fun idea. How are you doing the scoring?
My salon was doing pretty well until day three with the yin yang request came in. "Sorry, ladies. We're all going to be unemployed."
It works off taking a screenshot of the screen at the end of the timer, the pixel colours are comapred between the reference image and the nail with the relevant scaling factor. Each pixel in the nail is checked and the percentage accuracy is multiplied by the difficulty of the design. Hope that helped :) If you want more info, the code is very messy but I could upload it to github if needed.