It should be possible to hit rank S without ever opening every chest in game as long as you opened at least 90% of the chests that is.
Yeah, but I am worried about missing several exclamation marks because I do not have the space for more than 100 cards, potions, high potions or other items. Should I try to make some space and pick up the item or is checking the exclamation mark enough to receive the point.
I believe the points only count if you can open the chest and keep what's inside the chest but still selling cards if you own 100 of them still isn't needed you just need to open 90% of all the games chests for a S rank.
I always wondered, does finding CH&C treasures, as well as ocean treasures (the bubbly spots where you use a dead pepper) count towards your treasure rank? What about random items just littered around, not in chests?
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A master player can get rank S before even Ipsen's Castle. So don't worry. You can miss quite a few treasures and still get it. Chocographs and spots are the most important ones. As well as key items, I suppose.