The Belleville spring washers (.50 OD / .255 ID) that go on the upper steering shaft are missing from the pieces I got with my unfinished kit. I found them on the McMaster-Carr website (thanks to edwardb for sending me there) but there are four thicknesses listed. The choices are:

0.018
0.022
0.025
0.038

Because this is a "spring washer", I assume the thickness (height) makes a difference. What's the correct thickness to use?