The slight rotation of the castle nut with the split pin in place is sometimes enough to show no play (tight) or slight play (slack).
If you nip it too hard it will soon loosen off. Yourcstib axle may have two holes at 90 degrees, so check.
Or consider either a shim washer or facing off the nut with abrasive paper on glass to get to the next position.
A tiny amount of movement is ok anyway.
