Curious to hear what the pros recommend for this one. My first thought would be, as I think QB Junkie alluded to above, that you'd rather be operating with the lot #s in their own table than as a multi-select field on the ingredients table. Assuming I'm understanding your setup correctly. That way when you go to pick a lot number for a recipe, rather than using a multi select of a million potentially unrelated lot numbers, you use a record picker to the lot number table that's filtered by related ingredient.
If your ingredient/lot number relationship is many to many (ie. a lot number can be associated up to more than one ingredient, as opposed to lot numbers being strictly child records to the records on the ingredients table), it gets a little stickier, but I think you'd still start with a lot number table, do the standard many-to-many technique to a joined ingredient/lot number table and then work from there.
