Confused on cassette spacers

reborn2 Posts: 42
edited July 2013 in Workshop
Hey guys,

I checked out the cassette spacer thread in the FAQ but i'm still a tad confused so please bear with me.

I've got some Dura ace C24 9000's on the way. The tech blerb on the site says:
10 and 11-speed compatible freehub for use with cutting edge Dura-Ace CS-9000. (10 Speed spacer no included)

So for my shimano 10 speed cassette on my old wheels, i'll need a spacer. After doing some research I've read conflicting suggestions of using a 1.1mm or a 1.85mm spacer. Is there a set spacer size i should be using or is it more "play it by ear"? If so, how do i gauge the fit?

The last thing i want to do is damage my shiny new wheels with my limited mechanical abilities.