Not all motors need the exact same thing on collector length, HP, weather, track altitude, traction, header size and design and many other things can and will influence that
One of the old tricks was to use a crayon and make a mark along the entire length of the collector and see where the crayon melted and cut the collector off EITHER (CRS)1 to 2 inches behind or in front of that place
The time slip will tell you for that day also what works best, it wouldn't hurt to have several different sets with 6,12 and 18 inch lengths to keep in the trailer for future testing