Originally Posted by
Kontact
I doesn't hurt to go an buy some inexpensive used 26.6, 26.8 and 27.0 seatposts. All three will cost less than one caliper or sizing tool.
With nicer bikes I would caution against there being one correct answer when it comes to sizing. Stouter tubes for large sizes or mid year production changes could have a low production, high end frame receiving different seat tubes on otherwise identical models. The Opus III was top end.
That's a good strategy too. I already have a 27.0 and a 26.6 now, and have a cheap 26.8 on the way, so I have all 3 for future reference now.