Anyone have any good guidance on determining what size TT frame I need?

I tried reading through all the Slowtwitch stuff on fitting and looking up Stack and Reach, but it kind of left my head spinning. I thought the rule was to go down one size, so if I normally ride a 56, get a 54. But some of the 54 TT bikes I find have almost 56 top tubes.

Since the Masters Nationals are just an hour up the road the next two years, I figure I'll get a TT bike and start training on that too to give it a go at Nationals next year.