The 8th grade has $ \frac{650}{520+650} = \frac{650}{1170} = \frac{65}{117} $ of the total students. To simplify this fraction further, we notice that $ 65 = 5 \cdot 13 $. Since $ 117 $ is not divisible by $ 5 $, we test if it's divisible by $ 13 $ and find that $ 117 = 9 \cdot 13 $. Thus to have fair representation, the 8th grade should have $ \frac{65}{117} \times 18 = \frac{5}{9} \times 18 = \boxed{10} $ of the $ 18 $ representatives.