It will actually be 100%!
>Night Slash deals damage and has an increased critical hit ratio (1⁄8 instead of 1⁄16)
>Holder's critical hit ratio is boosted by 1.
>This Pokemon's critical hit ratio is boosted by 1.
According to this table, Night Slash will always land a critical hit.
Source: 1 | 2 | 3 | 4