Speedometer matching

[Mr Spigot]

I know there have been a lot of posts about calibrating speedometers with different gearbox/differential/model permutations and I seem to have spent hours reading most of them. However, I am still uncertain which one I need to be looking for to show a reasonably accurate reading although I am not expecting pinpoint accuracy

I am about to fit a 3.9:1 differential in my car which runs on standard 145 R/14 tyres. It has an MG Midget 1275 engine and gearbox. It is a 1960 car, so I would like to retain an original bronze face style speedo if possible, but it is not essential. Looking at lists of speedometer identification charts on this forum and the myriad of comments, it seems the TPM (turns per mile) figure is the most important and I need one that is either 1408 or 1376 to suit this application. As far as I am aware, the drive ratio in top gear is 1:1 so the gearbox and engine size should not affect this, but please correct me if wrong. I have come across a speedometer which is numbered SN4470/30 which seems to tick all the boxes, but I would welcome any knowledgeable advice here before leaping in.