The per diem rate for overseas travel has to be based on the company policy. Of course, this has to be in line with the RBI Guidelines. To know the RBI limits, please click on the following link to see the relevant circular:
https://rbi.org.in/scripts/FAQView.aspx?Id=66.
Company Policy for Per Diem Rates
Ideally, as a company policy, you must frame per diem limits for such overseas travel based on the grades of the employees traveling, the purpose of travel, duration of travel, frequency of travel, countries to be visited, and the cost of living there, etc. Even the authorization/approval of such overseas visits must also be specified. Having specific formats for such travel approval would also help streamline the request, approval, and release of foreign exchange.
Trust the above input meets your immediate requirements.
Regards