I didn't have a 4.7k handy so I used two 10k to get 5k for R5. With that I can get down to 40 or so.
Adding R8 and R9 as you suggest, the max I can get to the right is 460.
Edit: However, even with R8 and R9 disconnected, chaging R5 to 5k brings the max for the far right down to 680. R8 and R9 brings it down even lower.
I have a feeling that even if we get 000 on the far left and 999 on the far right, center is going to be somewhere down around 150 and no amount of adjusting those resistor values will put it near 500 without the left or the right being way off.
Adding R8 and R9 as you suggest, the max I can get to the right is 460.
Edit: However, even with R8 and R9 disconnected, chaging R5 to 5k brings the max for the far right down to 680. R8 and R9 brings it down even lower.
I have a feeling that even if we get 000 on the far left and 999 on the far right, center is going to be somewhere down around 150 and no amount of adjusting those resistor values will put it near 500 without the left or the right being way off.