Delta V = 1247-15 = 1232mV
Delta P = 49-0 = 49mB
therefore Dv/Dp = 1232/49 = 25.142mV/mB
zero offset = 15mV
Therefore output at 12mB = (25.142*12)+15 = 301.71+15 = 316.71mV
But, the Dv/Dp figure of 25.142 seems rather peculiar and certainly not what you'd expect to find in an industrial environment, where manufacturers endeavoured to use "round" numbers
Are you certain about your numbers?