Universal credit - claim back up to 85% childcare costs

Hi, I have a question about the “claim back up to 85% of childcare costs” So my son has just turned 3. He is entitled to 15 government funded hours nursery which he has been since he was 2. Once I start working, from September he will be entitled to 30 hours funded hours. I’ve got a job interview this week & any out of term time hours / additional hours he does over his 15 government hours UC have said I can claim back up to 85% of costs. I’ve seen so many posts that not everyone gets the full 85% claim back. How do UC work out how much of additional childcare costs you can claim back pls? For reference. I’m single, 1 child & live at home with parents. Thanks to any one who can help