Larry, You are posing some most interesting questions and I suspect you'll get several replies: all different. In order to go further you'd have to explain: why am I building this piece? Do I want to build a "replica" as nearly as is humanely possible, of an original period piece? Or do I want to make a nice piece "in the style of" or "close to" an original piece. Or do I want to build a piece using features of 18th century pieces but not necessarily following any individual piece. Or do I just want to build a table for my wife's porcelain collection, not being too concerned with what it looks like?
For the most part I want to be "close to" an original, or to look like a piece in a picture. In this case "scaled drawings" are a delightful luxury, but not a necessity. (Actually, scaled drawings are not necessarily perfectly scaled and even if so, scaled to what?) Scaling from a photograph is plenty sufficient though you will not be precisely accurate and you can easily add a half inch here, or cut a half inch there and keep pretty close to the "perfect proportions" that the Townsends and Goddards and others achieved. And you're right the phase of the moon might affect your efforts!