What would be the best way to calculate nutrition of a Gravy that contains one ingredient that is the by-product of a meat that is cooked in-house (ex: turkey cooked in house. remaining turkey juice used in gravy)?

By-products of other foods, like you mention here or broths, juices, etc., where part of the whole has been removed from the beginning ingredients can be challenging to calculate nutrients. If you cannot estimate the make-up of the juices from ingredients within the database, then lab testing might be the best option.