Proper seatpost diameter for 2001 Schwinn Rocket 88?

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 ;)
 
just to ad to the confusion, 26.8 mm came up when googling your question.

are you not able to get hold of a vernier callper and measure?

.... but just noticed your post states it hs been fitted with the 27.2 - if someone has forced an oversize post into the seat tube, it does not bode well for the seat tube. how tight is tight? the tube may be damaged so the correct sized post, be it 27.0 or 26.8 will now be too loose.
 
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