My dim memory of the Var 171 tool is that it was simply meant to be used to ensure that the relationship between the forward/back position of the nose of a rider's saddle and the center of the bottom bracket would be reproducible for setting up the rider's next bike (or the same bike after disassembly).

Just looked it up, and there's an illustration on page 134 of the 1972 edition of the C.O.N.I. "Cycling" book that shows how to measure that dimension for fitting a bike to a rider.

On the other hand, the illustration for the Var 171 tool does include a note indicating that the tool is to be used for checking the seat tube angle ("very important for frame building").