Persistent absence threshold is 10%
If a pupil's overall absence rate is 10% or higher of their possible sessions, they're classified as a persistent absentee. A 'session' is 1 morning or afternoon in school.
This is based on their individual absence level, not by comparison to a national threshold.
The calculation for this is:
[A pupil’s individual overall absence divided by a pupil’s individual possible sessions] multiplied by 100. If this is greater than or equal to 10, the pupil is classified as persistently absent.
Or, visually:

You can read more about this calculation in the DfE's guidance on pupil absence statistics, paragraph 3.2.2.