Well, looks like:
0-499 miles, -> 1
500-999 miles -> 2
1000-1999 miles -> 3, etc.

Multiply your 'Weight Rate' by your 'shipping distance value' (1, 2, 3, ... above).
Show result. Done.

Hint, the modulus operator might help you out in determining how many multiples of 500 miles you have.