Ordered 2.60 but CRC have sent me 2.58 have tried to ring but no answer.
Using this spoke calculator http://www.prowheelbuilder.com/spoke/index.php
Rear Chris King SS hub
Mavic 819 UST Disc
32 spoke 3 cross
Wanting to crack on but in two minds 😕
If it was me I would crack on... from my experience 2mm is about the most you can get away with.
(disclaimer... don't blame if it doesn't work 😉 ).
How long are the nipples? if there longish ones (14/16mm?) and theres plenty of thread in them then you should be ok.
Thats a point... do you not NEED long nipples for Mavic UST rims?
Oh aye, there funny nipples the 819 ones
Yes, you have to use 16mm nips (14 at a real push), because of the stupid inserts, also means a lot of spoke calculators get it wrong, the DT one didn't used to work properly for 819s IME.
Shorter spokes are lighter 
I dont have any paperwork here but 255/56 springs to mind anyways.16mm nipples too.
be surprised if it goes. Don't lace them or you may have trouble returning the spokes.