I am also doing AQA additional this year!
Looking at AQA's UMS grade boundaries, you need
at least 280 UMS points to obtain an overall grade B. You say you have 17% of your 100% (foundation).
17/25 = 68% of the full marks
So you got 34 (50 x 0.68 = 34) marks in your ISA, correct?? This equates to a low
C/D. This C/D grade is worth around
55 UMS points (a rough guess).
So 280 - 55 =
225 UMS points remaining
225/3 =
75 UMS points per exam (Bio, Chem and Phy)
You need to get
at least a mid B grade per exam.
Unfortunately, I must say it is impossible for you to obtain a grade B if you're doing foundation so the highest grade you can achieve is a C, even with full marks in the exams.
If you really want this grade B, you need to ask your teacher
ASAP to enter you for higher tier. Yes, you haven't covered higher tier specification but you
could learn it and you
do have a chance. Good luck!!