I'm in the same situation with a new RockShox Pike 409. I digg it all around for info and what suited me better was a solution like Saturnine explained.
Steerer tube lenght is 170mm, I have a medium sized frame with a 110mm headtube lenght. Got a normal stack height stem (40mm) and a CrankBrothers Headset (22mm). So I got 172mm total lenght with an aditional 2-3mm spacer between headset and stem. I'm hoping that the 5mm aditional space will be close to the 3-5mm mark that is needed to tighten the stem (stem above steerer top).
I'm gonna build this setup only in June so I have a long time ahead till mount and solve the problem. After that my other option is to get a low stack height stem, but I'm hoping that won't be necessary.