Speedo Cable length?

Without removing my Speedo cable, how can I find out what length to order a new one?

1962 Mark 2 RHD. Was a 3.4L Auto, now has a 3.8L MOD

Here is a place to start:

https://www.sngbarratt.com/us/#!/English/parts/f04b791a-812a-4c74-a680-bcff768a24f0

Gerard