I'm constructing a dual ring (40,28) and the rings are different brands. I know I'll need to add some ring spacers but I dunno what the official distance is. Looks like 'bout 10mm, does that sound right?

I'm pretty sure it's 5mm. Road double chainsets definitely are.

I just measured this on my XT chainset and its definitely more than 5mm… maybe 6mm. Its hard to measure though so I'm not sure exactly.
Anyone know for sure?

I reckon it's about 10mm

Someone must know the official measurement… anyone??

Sheldon Brown has some info – think it's on his chainline pages.

It's not a round number, I remember that much.

All I can find on there is he says shimano road doubles are 5mm apart. i think that is old info though – don't think he has anything relevant to current shimano cranksets, either road or mountain.

I doubt he's updating from beyond the grave 😆