Computes the total sample size required to estimate a population conditional mean of y at x = x* in a fixed-x linear regression model with desired confidence interval precision. The total sample size would be allocated to the levels of the quantitative factor, and it might be necessary to increase the total sample size to give the desired sample size at each level of the fixed factor. Set the error variance planning value to the largest value within a plausible range for a conservatively large sample size.