Do you mean with the CS-6600 Ultegra cassette (which is what I have)?

I went through this issue with Performance and with HED. Performance initially used 3.8mm on my son's HED wheels and that caused the 11T cog to shear off the end of the freehub splines, then spin freely. HED separately told me and Performance to use 2.8mm.

Here's a list of freehub spacer requirements from joejack951 from another discussion on freehubs:
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11 speed freehub + 11 speed cassette -> no spacers
11 speed freehub + 10 speed cassette -> 1mm spacer plus 1.85mm spacer
11 speed freehub + 8/9 speed cassette -> 1.85mm spacer only
8/9/10 speed freehub + 11 speed cassette -> won't fit (unless using a MTB cassette)
8/9/10 speed freehub + 10 speed cassette -> 1mm spacer
8/9/10 speed freehub + 8/9 speed cassette -> no spacers

I say 'general' because there are some Shimano 10 speed only freehubs with wacky spline designs that only accept certain 10 speed cassettes and don't require a spacer. Then there are Mavic road freehubs which have been ~35mm long since the early 2000s at least and for those the same rules apply as for typical 11 speed freehubs, even though Mavic originally only intended for them to be 8/9/10 speed compatible.
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Btw, I can attest to the above-mentioned the 10-speed Mavic being the same as an 11-speed. I just checked my 2006 Kysriums and they had 2.8mm of spacers (the cassette was already off, but spacers in place).

Also, back to the torque thing. Neither my LBS mechanic or HED would give me torque numbers. The mechanic said to tighten it until you get to the serrated edges (what!?), which is when it starts to slip/ratchet at the end of its travel. I'm probably not saying it right, but I can feel it when it happens. He does not use a torque wrench. I have one but need to buy a 3/8" to 1/2" adapter to get it to work with my cassette tool (which I'm willing to do if necessary). Opinions?

edit: just back from the store. Got the adapter. Next time the wheel is off, I'll check the torque on the cassette.