Look at the approximate slope curve under
You cannot expect a simple explicit equation for it.
A possible function is given here as dy:
X=[15 20 25 30 35 40 45 50 55];
Y=[11.89 11.845 11.85 11.855 11.865 11.93 12.01 12.2 12.77];
dY = gradient(Y,5);
dy = @(x) interp1(X,dY,x)
dy(22.8)
