Sourdough Apps...

This one is for Android

https://play.google.com/store/apps/details?id=uk.co.eastcourtmanor.sourdough&hl=en

This one is for iPhone 

https://itunes.apple.com/gb/app/sourdough-calculator/id463352431?mt=8

I use the one for iPhone quite a bit , comes in very handy.

Not sure if the Android one is for Phone or Tablet and I do not know if the iPhone one also works on iPads.