Cantilever stairs at Acorn Bank

The formula for the deflection of a cantilever beam can be found here https://en.wikipedia.org/wiki/Deflection_(engineering)#Uniformly-loaded_cantilever_beams The density of sandstone is 2000 kg per cubic metre. I'm estimating 150cm long, 10cm deep and 20cm wide. That gives volume 0.045 cubic metres and mass of 90kg. Weight is about 900N. Weight per unit length = 600 N per metre. Moment of inertia is width x height cubed/12 = 2.5 x 10^-5. I think it is this area moment of inertia that I need because the units work out correctly if done like this. The Young Modulus for sandstone is given as 1 - 20GPa so I'm going to choose mid-range as 10GPa. Putting the numbers into the deflection formula I get a deflection of 1.5mm. That sounds reassuringly sensible. I had wondered about the effect of the stair above pressing down but that must be cancelled out by the support offered by the stair below.