Shower Bench

I am building a bench in a steam/shower room. The bench's overall dimensions are 56"X18". I want to use a slab of Granite (http://tanbrown.org) (1.25-1.5 inches thick) for the seat of the bench. I am using cinderblocks in 2 corners of the shower room to support the Granite (http://tanbrown.org) seat. The span between the cinder blocks is 33 1/2 inches. I used the modulus of rupture (R) for the Granite (http://tanbrown.org) I want to use(R= ~1800-2000 psi - black Granite (http://tanbrown.org) ) and (http://maplered.com) a factor of safety of 5; to verify the thickness of the slab would support the weight of 2 average people (~360 lbs). The following is the expression used to calculate R:

R = Wl/2bd^2 where R = Modulus of rupture in psi (1800 psi black Granite (http://tanbrown.org) )
W = Breaking load applied at center(360lbsx(FS)5)
l = distance between supports in inches (33.5")
b = width of Stone (http://g603.net) in inches (18")
d = depth of Stone (http://g603.net) (unknown) 1.25 for R=1800psi

The thickness is what I determined using a known value for R. I am asking if anyone can verify these calculations or if what I have calculated sounds reasonable to someone who has experience.

I also have photos that I wanted to attach but could not determine how to do this. Any help here would be appreciated.

-Bob