That sleep library looks overly complicated in my opinion. If it's any help to you, take a look at a post I did last year that sleeps the 328p for multiple 8 second WDT intervals to achieve 1 hour sleep periods. It shows how the WDT sleep method compares to real time.
This is my own observation of one particular 328p and it will vary amongst other 328p micros. I suspect it will also vary on the same 328p as temperature changes.
Remember that the WDT sleep is a poor mans sleep timer to be used if you are happy with a 1 hour sleep being out by +/-60 seconds or so.