I bought an iPad Mini last week and have been figuring it out. Today I used it to display the photo to paint while using the Accuview app. You can see the grid and the value checker that is in the app. You can move the value checker around and resize it. I think it will take some practice to really use this tool correctly, but it was really nice to work from the digital photo..
I'm still enjoying creating my textured surfaces and this week found Peter Davidson's pastels online. He uses a homemade surface also, generally with a dark background. I decided to try this too, but using black gouache to tone my surface. You can see process shots following.
I really like how this one turned out. (I'm also trying out the Blogger app and need to figure out how it works!)