KARACHI, July 26 The Karachi Cotton Association on Tuesday fixed the official spot rate, or base price, for Grade 3 cotton at 6,698 rupees per maund (40 kg). Grade 3 cotton has a staple length of 1-1/16" and micronaire value between 3.8 and 4.9 NCL (no control limits), which represents fine to coarse classes of cotton varieties. In the kerb market, the key crop varieties traded in the range of 6,450 to 6,700 rupees per maund. The following are the grade and staple margins for July 1 to July 31: GRADE 31/32" 1" 1-1/32" 1-1/16" 1-3/32" 1-1/8" Super n/a n/a n/a n/a n/a n/a 1 n/a n/a 107 214 348 n/a 2 n/a n/a 27 134 268 348 3 n/a -536 -322 BASE 134 214 4 1125 -804 -589 -268 -134 n/a 5 -1340 -1018 -804 n/a n/a n/a (Reporting by Syed Raza Hassan)