Grading for PPE

Hi there,

So recently my school has done Paper 1 of the 2020 Combined Science Papers. I've received my Physics result which was an 18/70 (25%) and my Chemistry which was a 40/70 (57%). I haven't received my bio yet but I decided to look at the November 2020 Grade Boundaries and found out I got a grade 5 for physics and a grade 8 for Chemistry however I want to know how I can work out my overall grade for combined science.

Could someone give me a rough estimation of what my overall grade would be for Physics and Chemistry? Would it be an 8? Would it be a 7 or a 6?