Your average (if you measured correctly) then becomes 66.756.
So...the next jump is 1/4 mm, or .025 mm (depending on piston manufacturer).
.025mm = .010 inch, or ten thousandths of an inch.
Depending on the "out of round" of the bore, I personally, ....would safely say ... that a 67.00 mm piston of your choice would clean up the bore after machining.
That's a .005 (5 thousands of an inch) "Swipe" all around the bore. This will be enough to straiten and make the bore round again. This is dependent upon any deep gouges or grooves you may have, that need to be removed.
I only seen one photo, which wasn't enough
so when i order a new piston it would be 1.00 over bore? because stock bore was 66.00 so one over would be 67.00?