Dear Abhi,
Let me tell on the off-set, that there is no thumb-rule on calculating the raditions when engaged in radiography in construction. BARC is the only agency that decides, the strength, the emissions strength and the radius of affectation. The agency that is involved in conducting the radiography, is given a license, dosimeter, a table with the specific criteria of the radioactive substance used in the camera, alongwith the strength of substance and the distance the radiation covers. This information can only be given by BARC and no other authority is authorized to handle or conclude on this topic. So, please refer to your agent's documents, and you will find the table that will exhibit the details of isotope and the precautions that are to be taken.