Read post 35 in this thread:
http://thefiringline.com/forums/show...+spacer&page=2
Post 33 in that thread explains why.
I've done this and compared readings to expensive case headspace gauges. Best price-performance ratio I know of to measure case headspace to set full length sizing dies to bump fired bottleneck case shoulders .002"; about perfect for long case life and best accuracy.
Note the standard reference diameter on .308 Win cases (chamber too) is .400" but for just measuring how much fired case shoulders are set back by sizing, that nylon bushing's 3/8" inside diameter will work perfectly.