Cassette spacer confusion..

I have just put on a new ultegra 6800 wheel and it came with a 1.85 spacer for 10 speed cassettes, which is what I have, (5700). I put it back together and it was fine with although I had to tighten the cable a bit. I then remembered the 10 speed spacer you get with the cassette so I checked Simano tech and it says to use both although no mention of it in the paper instructions. So I took it off again and added the spacer but then I could only just fit the last sprocket on and there was no way the lockring was tightening up, not even close. I put the cassette back on with just the 1.85 spacer and it seems ok. So is there a definitive answer re. the spacers or is it just a case of going with what works ?