I suggest that you start with DOE > Custom Design. Define your response. Define your continuous factor. Define your model terms. Try some number of runs, make the design, and then open Design Evaluation > Power Analysis. You have two choices here. You can specify effect size and random variation on a relative basis. Leave RMSE at 1 and enter coefficients as multiples divided by 2. So if you expected an effect to be 3 standard deviations, enter 1.5 for the anticipated coefficient. Or you can evaluate it on an absolute bases. Enter the actual RMSE (response standard deviation) and the coefficient. This value will be one half of the total change expected in the response over the factor range.
