Well, if a bundle of 8 once was 100 feet total, then you don't need to guess - each length would obviously have been exactly 12.5 feet.I dont remember but id quess 12.5 feet, back in the day you fitted about 8 bundles a day and all your measurements were done in your head, so round numbers helped, similar i suppose to how Brick sizes originated to help builders.
However, as I said, 12.5 feet is just over 3.807 metres - so when the 'standard length' of 3.75m arise?
Kind Regards, John