What you are looking for is just a linear fit to your exponential function, you can do this by using the mldivide operator "\".
phi = 2;
n = 100;
theta = (1:n)'/n;
f = (1-phi)*theta.*exp(-0.67*theta.^1.5);
fOfA = @(A) A*f;
measureddata = fOfA(2)+.05*rand(n, 1);
coeff = f\measureddata;
plot(theta, measureddata, theta, coeff*f)