Sam,

I cannot tell about the exact origin of the Indian Boiler Regulation IBR-350 Formula, but can throw a light on the probable sources.

The current code ( American Codes) formulas area result of work done by
Bailey-Nadai,Barlows, Boardman , Lame etc.Based on what you have mentioned it seems that they are probably based on Barlow's and Boardman's approaches.

For your second question, I would like to mention the use of Larson Miller parameter. It is not possible to give a full description of the Larson Miller Parameter as it is quite a lengthy one. I would advise that you go to google or yahoo and type "Larson-Miller Parameter" in search. You will get ample information on this subject.

Regards