D4 = 1 + k*d3 / d2 where d2 and d3 are unbiasing constants, dependent on sample size used to calculate the range. D4 is often tabled for k=3. For other values you can use the formula provided you have values for d2 and d3. For a moving range based on 2 samples, d2 = 1.128 (as you already know), and d3 = 0.8525. If you calculate D4 using these values and k=3.5 your calculated moving range UCL will agree with JMP.