Sizing rod tips..

I'm trying to replace the tip on my baitcaster and unfortunately my local
tackle shop didn't have any tips in stock. I checked out Sports Authority
and they had the tips but I didn't know what size to get. I spoke to the
person running the fishing section and asked that I bring my rod in. He
gave me the size he thought was right but unfortunately, it wasn't. I can
fit the previous tip into the hole of the new tip. So, back to Sports
Authority tomorrow...

So is there a way to tell? Is it just an eyeball game? Should I just order
the 3-pack from Fuji and hope that I get the size I need?

Christopher