IF: Could RTN costs be used for a top up to the salary of the researcher? If yes, which is the best way to do it - as a monthly top up or at the end of the project?

Yes, it is possible to use RTN costs for a top-up. The method is up to the host organisation and should depend on the circumstances and their normal practices. For one, it could be difficult to know beforehand how much of the funds will be unused, so it could be difficult to determine how big the monthly top-up should be. On the other hand, a large one-off payment could influence the researcher’s taxation regime.