Eligibility for tax free childcare and 15 hours free childcare

Hello, I would like to get some clarification on how eligibility for tax free childcare and 15 hours free childcare is calculated. At the moment I earn over the £100,000 threshold to be eligible. But I would like to understand, if I use salary sacrifice to earn under £100,000 , if this would make me eligible and most importantly, how is it calculated? For example at what point of the year would I need to start my salary sacrifice in order for me to be considered as eligible? Thank you in Advance

Hi Thomas,
For child benefit advice, contact our Child Benefit department via Child Benefit: general enquiries.
Thank you.