I bought 20 pound catches. When not mounted on a door, and the catch plate mated perfectly to the magnet, perfectly aligned and flat in all six degrees of freedom, and the plate is pulled perfectly away from the magnet, in a direction perpendicular to its face with equal force from both ends, I estimate the force required to be about 20 pounds.
When mounted on a door, with the magnet on the top stop and the plate on the inner door face .... it's a different story. The door and the stop are not perfectly perpendicular in either of the two axes where they need to be. The plate does not mate perfectly with the magnet no matter how carefully you adjust it.
The mounting screws interfere slightly with the mating of the two surfaces.
Opening the door does not pull the plate in a perfectly straight line perpendicularly away from the magnet. The door twists a little and the plate comes away from the magnet with unequal forces all around its surface.
All this is very subtle and slight, but the effect is to reduce the opening force from around 20 pounds to around 2 pounds.
2 pounds is perfect for a well aligned door with no warp, no swelling or problems in the hinges, and a closet that is not overstuffed with clothes that are pushing the door open.
For all normal closets however, that have at least one or two of these problems, I would say a 40 pound magnet might be a better bet and if you want to provide any resistance that a user would think of as "strong" it would need to be a 90 pound rated one that would require a very strong pull from an average person.