depends on the SSD and how you partion it.This is backwards. You should NEVER let an SSD have less than 20-25% space, as they only have a limited amount of R/RW cycles and thus the increased read/write of finding shit around clutter causes a hugely acclerated burnout rate. A HDD is nominally fine up to 10-15%.
I mainly use enterprise ssd's that have more spare memory cells.
And when i use consumer stuff i shrink the partion to 80% of max capacity of the drive to create more spare area.
but a hdd that is above 80% will start the get very slow as you are getting to the edge of the platter with long seek times.