Not sure about M48s but you can buy packs of very thin nylon spacers, as thin as 0.25mm, 0.5mm. 1mm, etc. I bought mine from Teleskop-Express. They are made by Baader. They simply slip on over you adaptor threads, to tune exactly the length of spacing you require. Link here.
https://www.teleskop-express.de/shop...ol---15pc.html
BTW, most Correctors have a working range, and do not necessarily need precision like 2,56mm, they could work well 50% either side of that probably.