I trust hammockgear ratings, but I still add 1oz just for peace of mind. The "already overfilled" thing is a gray area for me, like the "25% MORE!!" label on a shampoo bottle. 25% more than "before"? The competitor next to it on the shelf? If so, which one? The 10oz bottle? The 12oz bottle? The 16oz?
I feel like the +1oz option settles any questions I may have for a modest $8.
Bookmarks