[senco-forum] LSA Staff per secondary School

Dear Colleagues,
sorry if this has been asked before.
I wondered if there is any kind of guide to how to calculate the number of Learning assistants or full time equivalents required to support pupils at SA, SA+ and Statements in a Secondary school. I realise the number required for pupils statements is obvious, but wondered if there might be a formula for the remaining pupils. I was wondering how to estimate the required numbers and whether it could be linked to pupils on the SEN Register and their needs etc. the reason for asking is that I am planning support for next academic year and am new to the position.
If anyone could help or has any advice on how they did it I would be grateful.
thanks
Mark
SENCo, Secondary, London