I consulted out local Smiths man today. He was trained by Smiths, bought all their gear when they left Australia, and is in the throes of retiring.
He said...
The inner should protrude no more than 7/16ths out of the top of the outer when the crimp is pushed back inside the outer as far as it can go.
At this point the inner should stick out the bottom of the outer by 5/8.  He made the point that the 5/8 sticking out all needs to be square, if some of it is still round then it will push the inner up into the speedo/tach.
The consequences of the inner protruding too far into the speedo/tach is that it destroys the mechanism.

So those are the measurements all new cables should be checked against.  He says many new cables are wrong, and despite his broadcasting the  needed measurements locally, the over long cables that turned up in the post Smiths era supported him very well for years by causing a lot of broken instruments.

He also mentioned the flick test for new cables. Cables, he said, should never be coiled tightly, but left hanging. the inners take a set and this causes a flick in the cable operation leading to wobbly needles.
When people say, it can't be the cable, I've just replaced it, he says, yes it can, and shows them the flick test. This is a Smiths test and is laid out in a pamphlet he gave me a copy of. What he referred to as "flick", they called "snatch".

It goes like this... pull the inner out of the cable and hold it in a
loop, one end in each hand, pointing up and held like you would hold a straw.  Gently roll one end between thumb and forefinger and feel wth the other hand whether it starts to turn immediately. If it does it is OK. When they are not OK they don't turn for a split second and then flick around, as if something is bent.

He showed me brand new cables that did it. So there is the inner outer length thing laid out... I would never fit a speedo cable without checking the inner length because I have done it, and it hurts.

regs
Chris