1. Sec 44AD deals with presumptive taxation for eligible assessee carrying on an eligible business. Job work is an eligible business and presumptive taxation u/s 44AD applies to Job work provided Gross receipts or T.O does not exceed Rs. 2 crores.
2. Presumptive taxation scheme was introduced in order to ease off difficulty in maintaining books of accounts and other compliance issues. So it is not mandatory for an assessee to opt for presumptive taxation.
3. In your case, if the assessee wants to show a profit as Rs. 3,15,000 by properly maintaining books of accounts (by claiming all the expenses) it can be done.
4. The point to be noted in here is that according to sec 44AD(4), in case if the assessee has opted for presumptive taxation, it should be followed for 5 AY succeeding the AY in which the scheme has opted. On failing which the assessee will become ineligible to opt for presumptive taxation scheme for the next 5 years.
Please correct me if the above solution has an alternative view.