Minitab calculates the planning estimate for the parameter you want to estimate based on the planning values and distribution that you provided.
For the insulation data:
You plan to estimate the 10th percentile.
The calculated planning estimate for the 10th percentile is 393.094. Minitab calculates this estimate from your planning values for the percentiles. That is, for a Weibull distribution with 12th and 20th percentiles of 500 and 1000 respectively, the 10th percentile is 393.094.
The target confidence level is 95%. Because the resulting sample size must be rounded to an integer value, you cannot achieve a confidence level of exactly 95%. Your actual confidence level is slightly higher than the target confidence level.