Storm Giant Warhammer is a good weapon for knocback testing since it has less to none for stunning. Since zombies has a fixed trait, even if you choose a different weapon to test, the ratio will not differ drastically.
The "units" itself actually is based on ratio, but since it may differ like penguin said especially in knockback resistance so think it as a "rating" number from 0 to 10
But don't worry the rating won't differ drastically except for several weapon that have "pure" knockback like Magnum Drill, Reaper, Gae Bolg, and Triple Barrel Shotgun
For weapon that have "pure" knockback itself still can be reduced by knockback resistance from zombie but very little so it's very hard to be noticed
You can test it by yourself in Playroom, but still don't think this data is perfect 100% accurate since there's a lot of thing and factor can affect the stats itself. Just think this data as your base data to know your favorite zombie or enemy power not advanced one
Still though it's very hard to measure speed and stun resistance, since I checked these 2 stats every zombie in playroom. The result is almost no difference so probably I need to find better indicator and method to measure it