If it's a really tight fit (ie. doesn't move up and down fairly freely with the clamp undone), then the chances are you need a 27.0.
Funnily enough, I have the opposite issue with my Schwinn 4 Banger. It has a 26.8 post fitted (which is supposed to be the correct size), but the clamp has to be done up a fair way for it to hold properly. I tried a 27.0 post in there and, although it was a bit tighter, it seemed to fit a bit better..
I'd just get a cheap and cheerful 27.0 post (they come up for sale on here pretty regularly) and see how you get on with it. If it turns out to be a better fit, then you can invest in a better post