Is there a uniform rule on how grades from abroad are converted?
The new guidelines of the European Commission, which have been published in the ECTS Guide, have been in force since 2009 for the transfer of grades of study credits earned abroad. You can find the ECTS Guide here. Further information from the European Commission on ECTS can be found here.
Most partner universities provide information on their grading system and conversion with the final Transcript of Records, which you can use as a guide. It is best to discuss in advance with the Erasmus coordinator of your department how many ECTS the courses you attended from abroad are worth.