1nm for IRT anyway as said above. Have heard this was simply due to some DME's only doing whole miles, but I have never seen one myself so don't know.
FWIW I think 1nm is a massive tolerance, I am no hotshot by any means but either I have a knack for DME arcs or it's a generous allowance, because I used to be disappointed to not keep it within +-0.1 when doing my IR.