There is a guidance document, from the NUT, that sets out how they cnsider SENCO time should be calculated.
Formula for guiding Senco Essential Duties Time (per week) (NUT Charter 2006)
The following objective measures were used in devising the formula to act as a guide for allocating Sencos an appropriate minimum number of hours for their essential duties.
A) The notional SEN budget allocated to the school to the nearest thousand (eg, School X has a notional budget of £67,755 = 68 points)
B) A points allocation based upon the PLASC data as follows:
a) Number of pupils at School Action (x 1)
b) Number of pupils at School Action Plus (x 5)
c) Number of pupils with Statements (x 5)
(School X has 10 pupils at SA = 10 points; 5 pupils at SA+ = 25 points; 6 pupils with statements = 30 points making a total of 65 points).
Adding A + B = 133 points
Every 40 points = 1 hour so the Senco for School X is recommended 3 hours.
C) Pupil mobility – annual pupil turnover is both a major factor in, and an indicator of, the workload of Sencos. Thus an additional factor (based on a local authority’s annual mobility data) has been built into the formula thus:
Pupil turnover greater than 20% = 1 additional hour
Pupil turnover greater than 30% = 2 additional hours
Pupil turnover greater than 40% = 3 additional hours
School X has a pupil turnover of 34% and thus the Senco gains 2 hours making a total overall of 5 hours.
but this is just guidance!