There’s a thread where some good measurements of the various belt tension moduli were taken;
The simple output of that is that, for you, assuming they were fibreglass not the polyester belts the steel cored belts are approximately 70-75% as stretchy for the same force.
If you’re going to be retensioning your belts, I’d recommend measuring the belt tensions as you do it, recording them so you can check them for changes later and balancing your Y axis belt tensions to get square movement;