How to decide on the correct number of clusters?

clear
load fisheriris
figure(1)
Z = linkage(meas,'ward','euclidean');%Create a hierarchical binary cluster tree using linkage
[Hr,Tr]=dendrogram(Z);  %generates a dendrogram plot of the hierarchical binary cluster tree

val0=eval(vpa(Z(:,3),5));

t=1; n(1)=0; val=val0(1); f(1)=val;
for i=1:length(val0)
if val0(i)==val
n(t)=n(t)+1;
continue
else
t=t+1;
val=val0(i);
n(t)=1;
f(t)=val;
continue
end
end
figure(2)
fsum=cumsum(n);
plot(length(val0)-fsum, f, 'o-','LineWidth', 2)
xlabel('Number of Clusters')
ylabel('Merge Distance')
title('A sample evalution graph')