Shimano 11 speed MTB cassette on an 11 speed road hub, which spacer should I use?
For 11s MTB cassette on road 11s HG hub, use 1.85mm spacer.
Bike-Components.de lists the following:

1 mm: Thickness: 1 mm Application:

using a 10-speed (Road) cassette with aluminium spider (e.g. CS-7800, CS-7900) on an 8-/9-/10-speed freehub
using a 10-speed (Road) cassette on an 11-speed (Road) freehub (1 mm + 1.85 mm) Manufacturer Part Number: Y-1Z807000

1.85 mm: Thickness: 1.85 mm Application:

using a 8-/9-speed cassette (Road) and an 8-/9-/10-/11-speed cassette (MTB) on an 11-speed (Road) freehub
using a 10-speed (Road) cassette on an 11-speed (Road) freehub (1.85 mm + 1 mm) Manufacturer Part Number: Y-4T724000