[ATM-Buddy] Addition of spray silvering calculator

You can find ATM-Buddy, my small collection of mobile tools to make telescope mirrors, here : https://lucassifoni.github.io/atm-buddy/

I incorporated Gehrard S.’s recipe in my calculator, with color codes to avoid errors in handling.

Source code is at https://github.com/Lucassifoni/atm-buddy . Feel free to fork & adapt to suit your needs.