Basic principles in calculating moments in slab

There is a Reinforced slab of 300mm thickness,6250 width and 17900 length.The cover is 50mm from top and bottom. The reinforcements are 16mm dia. there are strain gauges attached 50mm from each surface fibre(top and bottom)reading strains in concrete. the interest is to study the concrete behaviour under thermal loads from sun. How do i calculate stress from strains and maximum forces from stress and maximum moment in the slab from force and lever arm. stress distribution is triangular. I have attached hand calculation example(the strains are maximum strains in the year 2000), is it the right approach? If not, how else do i calculate maximum moment?