30 free hours reconfirmaton

Can anyone find anything that confirms the £100,000 threshold is based on the tax year rather than at the point of signing up/reconfirmation - at least this is what I was told when signing my child up for the 30 free hours.

I guess we are in a unique situation in that my wife was recently made a partner (moving from a salaried role). This means that her income for this tax year will be less than £100,000 but her income for the next tax year will likely be over £100,000. As I understand it we are able to claim the 30 hours up until the April term.

The other added complication is that as self employed in a partnership she doesn't necessarily know until the tax calculations have been done whether her adjusted net income will be over £100,000 - does anyone know what HMRC advise in this scenario?