adapter shim for Progressive 412?

[baggins]

my stock shocks have lost all damping so I am planning to replace them with Progressive 412HDs.

their website says I need an adapter kit or a 14mm washer to ensure proper alignment, but they don't say how thick the washer needs to be.

I can save $40 + whatever shipping is if I buy on amazon instead of directly from progressive, but it seems the adapter kit is only available directly from progressive.

any idea how thick a washer I need?