You would likely find a rate chart useful. It will explain the major rate changes in 1845 and 1851, distances, sheets/weights, etc.
You note several as "high rates", which are often a combination of sheets and distance.
The Walworth NY to Kingston NY cover (not Mississippi) is a double-rate cover, rated at 18-3/4 cent for 150-400 miles.
This cover which you sold on ebay several months ago was a prepaid 4x rate for the 6 cent rate for not over 30 miles. Someone snapped it up via BIN quickly as 4x covers are less common, especially prepaid. I wish I had seen it in time. All I could do was save your images.