My way (5 cents) …
If the derivation is done in the EDC system and the investigator can inspect the result, then I do not set DRVFL to “Y”, i.e. I consider the data point as “captured”.
If the derivation is however done during SDTM generation, I definitely set DRVFL=Y. I presume this is the same as saying “when a record is added comparing with the source”.
BTW, this is a question that is better asked at e.g. the LinkedIn “SDTM Experts” forum (LinkedIn Login, Sign in | LinkedIn).