How much oil did you observe coming from the drain mast..??
I don't have the FCOM at hand so do not know what instruction was placed for you to comply with - call the engineer I guess..
Being practical the oil leak would have to be positive and not a weep or stain, before I run the engine.
The oil will come from the cavity but could be either gearbox or IDG seals, both have a limit of 3 d/min or 10cc/hr.
Shouldn't be over servicing as the servicing of the IDG will have a fill tube and a drain tube attached to ensure the correct quantity.
As mentioned, after an engine run seals will be bedded in and if it was statically leaking no oil will show in the collector can attached to the drain mast, then good to despatch.
Even if you were worried about this oil dripping, 25hrs or 10 cycles should get you home after re-checking the IDG oil level.
AMM 71-70-00-600-010 for light reading..
Finally call Maintenance Control if the leak was well over limit, as 6 d/min is still 20cc/hr and depending on your flight range will get you back to base for final fix, unless you are stuck in Bali.....
