Originally Posted by Bayner
Hey, don't worry. This is ADV, and nobody should be such a tightwad as to take offense that easily!
The length of the spacer needs to match the distance between the innermost bearing seat surfaces. Exactly the same would be ideal. The clamping force of the nut is to keep the axle positioned in the swingarm (and obviously to ensure the nut doesn't back off). It is not to 'preload' the bearing assemblies. The spacer should be in contact with the bearings prior to tightening the axle. (Spin one side bearing and the other should turn with it.) Only one bearing is held in by a circlip, so the appropriate spacer length is less of an issue than the depth the second bearing is seated to.
So maybe a little test would be to torque down the axle with the wheel off the bike. Put some spacers on the axle about the same thickness as the swing arms. Then you'll be able to turn the axle by hand and see if there's any sort of binding, which should show up if the bearings are preloaded.