[unruhjonny]

I seem to have observed that they are often not installed that far back (3.38" or roughly 3-3/8").

BUT(!)

These decals are also on cars which have been restored, so I have assumed that the owner did as you did - and moved it forward some.

I do not know if the assembly manuals ever gave a different location, but that measurement isn't too far from the forward edge.

I just took a rough measurement on one of my hoods (it's behind stuff), and it looks like the forward edge is about 1.75 (or the better part of two inches) from where the fibreglass on the hood turns up to form the scoop- so the diagram is suggesting that the forward most edge of the decal should be a little over an inch back from where the fibreglass turns up.

Again, I would assume that this measurement was a nominal number, and intended to tell assembly line workers approximately where it should go - but not be a real world hard number of where they were installed - unlike a badge, where I would expect a jig of sorts would have been used to drill pilot holes.