Looking at the graph in figure 4 of the linked datasheet I would suggest using mapping as: KPA = map(analogRead(13), 0, 1023, 0, 260);
That looks to give a reading about 9% low. I was going by page 2 of the datasheet that said the min was 20 and max was 250. I understand what your saying about the graph though.
Out = OutMin + ( OutMax - OutMin ) * ( float(In) - InMin ) / (InMax - InMin) ;
I tried to work this out but I guess I'm not clear on what is In and what is Out? Is In 0-1023 and out 20 to 250?