1st oversize for a TS250/1 is 69.50mm. There’s no point in removing more than you need to so, assuming you don’t already have a 70.00mm piston and the bore will clean up at 69.50mm, I would buy a 1st oversize rather than going as far as 70.00mm.
FWIW, I specified 0.05mm clearance for mine. Unfortunately, the RAM piston I had bought turned out to be poorly-machined scrap and so I replaced with an Almot. This was discovered *after* I had the barrel rebored and the Almot was ever so slightly smaller so the final clearance was closer to 0.08mm! Not far from MZ’s “worn out” figure, but it runs nicely and at least I didn’t have to worry too much about it nipping up during running in!
By the way, the best price I could find at the time for an Almot piston was at Kultmopeds.de