Yes, that is your best bet. As a rule of thumb, for Japanese frames;

hi-tensile steel - 26.2mm and under
plain gauge CrMo - 26.2 - 26.8 mm
butted CrMo or manganese alloy - 26.4mm and over

Unfortunately there is some overlap, so there is no definitive answer if you are in the grey zones. And Araya is not one of the major manufacturers, so they may have pushed the grey zones a little further. Please post your findings for future reference. TIA.

Edit: Don't forgot to check if an undersize post has been used. This will give a false measurement. The easiest check for this is to ensure that the binder slot in the back of seat tube is not narrower at the top, when the post is in place and the cinch bolt is tight.