Yield variances

Hello Sir, I'm getting stressed about this particular issue please help me out. I've watched all your Mix and Yield variances lectures and even worked through them with you. However reading the bpp text and working through its examples I'm super confused about: Whether to use 'standard cost of each material' or to use 'WEIGHTED AVERAGE COST/Kg' of each material to multiply the yield variances in (kgs).... Only if I multiply my individual yield variances with weighted avg cost/kg I get the correct answer otherwise not.