Design strength is calculated from the estimated Weibull parameters. You need to find the percentile (1% usually) at a given confidence level. For example, if you find the 1% percentile with 99% confidence, then you can be 99% confident that 99% of strengths will be above the design value. The sample size affects the estimated percentile. For 2-parameter Weibull, I published a paper (co-authored by D. Hudak) that you can download from my list. For 3-parameter, Military Handbook 5, Section 9 (you can find it online) outlines the procedure. Good Luck!