Can you provide the calculation you use to determine the weight result? With 50-75 possible entries, a single calculation (if there is consistency) can be used. But if not, then I would suggest another table with the three fields (size, weight and result weight).
Option 1: Use a Case function
Case ( size = 22 and stock = 30 ; 34.76 ;
size = 24 and stock = 30 ; 37.92 ;
size = 26 and stock = 30 ; 41.08 ;
size = 22 and stock = 35 ; 36.88 )
Option 2: Set up lookup that looks up the stock weight from a table of records where you record size, stock and weight in individual records. Your relationshp would match both the size and stock fields in your Parent table to corresponding size and stock fields in the look up table.
Works like a top! Thanks,