Dishonored Guide: Opening Doctor Galvani’s Safe in Overseer Campbell Quest
This safe is on the first level (second level if you are from America) of Doctor Galvani’s house in the main living/dining room:
The combination is hidden on the same level in Doctor Galvan’s Journal in his sleeping room which is to the right of the stairs that lead to the first level. Read the journal on the writing desk and you find out that the 28/7 is Galvani’s most important day. 287 is the combination for the safe.