if a site is letting you fill a co2 bottle yourself, they should be shot.
There is a certain skill to filling co2, it's not just open until it fills no more.
As others have said it's done by weight, and if i remember right a full 20oz bottle weighs in at around 3lbs 4oz, if you want to avoid any liquid issues.
You have to account for the extra weight of the fill rig and lead, i think mine would weigh 3lbs 10oz when full with the rig connected.
Going back to the first line......get a qualified person to do it.