A couple of references that I keep on that subject 1) Kent ME handbook on power 2) Babcock & Wilcox " Steam its generation and use" and I am sure that there are several other references that you can review to suit your situation. Since your question is too general, it would be useless for me to present their formulae due to the various conditions that contribute to such losses. Also boiler manufacturers do provide technical literature on their product, see if you can get a copy specifically on that subject.