Lets take your item (236 pearls) as an example. Your Blacksmith is at level 11 and have 6 slot. That means that during the event, you can melt 20.6 pearls an hour or 61.7 with the boost on.
Lets say you started the item with an hour left to your boost. That mean when the boost end you will have melted 61.7 pearls. That means it will be left with 174.3 pearls to be melted.
At you normal event pace, it will take an additionnal 8 hours and 27 min (174.3 / 20.6) for a grand total of 9 hours and 27 min. The numbers you have on your screenshot is accurate. When you will start your boost again, it will show a smaller number.