If you enter missing values for Number of Whole Plots or Number of Subplots, JMP chooses values that maximize the D-efficiency of the design. The algorithm uses the values specified in the Split Plot Variance Ratio option. See Advanced Options > Split Plot Variance Ratio. The D-efficiency is given by the determinant of, where V -1 is the inverse of the variance matrix of the responses. For further details, see Goos, 2002.