How to solve the vault in chapter two, give an explanation not an answer?

Go to the restroom using the WC key found on table in conference room.

Try the sink, water won't come. Now go to the loo. Use the screwdriver stuck in the vault to open the vents and get a key. Also, remove the painting opposite the vents to reveal the main valve. Turn it. Now try the hot water in sink. This will generate steam. Zoom out and you will see a wall clock impression on mirror. Remember it and reproduce it at the clock where time is money is written. A code will appear. That is the vault code. But the vault is broken so it types 4 when 3 pressed, 8 when 7 pressed etc. Enter code and open vault