You don't want to use a crush washer only with the brake indexing if accuracy is important. Anytime you impart more than 5 foot pounds of torque at the muzzle brake or flash hider on an AR, it will effect the bullet as it passes the muzzle. For duty weapons, not a problem, but for accurized ARs, this is not a good idea. Sometimes a crush washer will index it correctly, but usually not. The shim route is the way to go, especially with OPS Inc brakes. KAC makes a shim set that usually comes with their SOPMOD flash hider. If you call KAC, they may just give you a set.
For the install, use the crush washer and a shim or combination of shims so that the brake will be just finger tight at the 11 o'clock position, then wrench it to 12 o'clock (the thin web of the brake at 12). That will take about 5 foot pounds to do from finger tight. Loctite it for final install.
This technique is developed and used in building real, deployed SPRs.