Out of boredom I just watched a you tube video on changing the timing belts on a Ferrari 355. When it came to tensioning the auxiliary belts (alternator, ac etc) there is a special tool that Ferrari use to measure the tension. The tool was expensive and so when the job was complete and the belts tensioned correctly, he used a free app from Gates which measures frequency on specialised bike drives to record the frequency of a correctly tensioned Ferrari belt. This can then be used by any Ferrari owner rather than them having to buy the special tool. It’s simple to use, you just twang the belt like a guitar string and the App shows the frequency. I see no reason why this can’t be used on any engine with manual belt tensioning provided the correct frequency has been published by a kind user. Useful if you don’t have a belt tension measuring tool perhaps. When I am next to my spare engine, I’ll do some back to back comparisons on my VX timing belt and publish the frequency target if it works..