you need to look at where the installation is in the feild i.e access points/ turns/elevations then leave site closures at these critical points.
e.g. install the straight lengths in the piperack then when it turns or a bend is in place just do a site measurement give this to the fabricator then the spool will be fabricated and you know that it will fit.
leaving ffw is the best way to ensure that the installation goes without any cock ups.
regards