What spacers to limit downstroke on 501x shocks

[kaszal]

This is more about general damper set up question but I've got 501x shocks fitted to my Schumacher Procat so I thought I post here (got good replies before) I need to reduce the downstroke at the front so I'm going to put spacers under the pistons. The question is, does it matter if these are o-rings or alloy spacers? I presume alloy spacers are more precise. What do you guys do? Any advice appreciated.