Different types of Met msg - CM, BM, - are produced from raw sounding and measurement data data, the data in the msgs includes ballistic weighting factors. The lines in CM and BM don't always align and the NBC met msg has, I think, unweighted data. FTs tabulate corrections for the variations in wind, air temp and density from the 'standard conditions' (for which the FTs are compiled, and is the ICAO standard atmosphere - apart from charge temp!), these variations are provided by the met msg. Basically it's merely a matter of multiplying the difference from standard in the met msg by the correction given in FTs. IIRC US FTs provide instructions, perhaps this is no longer the case.