For consistency between city and hiway mileage, I like to refill at around 120 miles on my RKC (5 gal tank). I have found that the light and fuel gauge are accurate as to end of fuel supply, but very much inaccurate as to percent fuel remaining. The guage shows linear readings but I have found that the actual readings of the gauge should be changed: larger space between markings when full, ging to much smaller space between markings as you approach empty.
I use the trip mileage as primary indicator of how much fuel remains.
Enjoy,
Rich P