HELP! I'm having trouble measuring my spa for a new cover!

[SurgTec]

Okay - I was doing great until I got to the part about 'radius' corner dimension.  I have a Marquis Quest Spa - circa 2005 model.  It measures 77" x 84" and has curved corners. 
I've tried the two rulers, a ruler and a straight edge, a carpenter's t-square and the tracing and measuring methods - I came up with a different radius every time! (everything from 12" - 15.5")   I understand the importance of getting the measurement correct - and I even called one cover manufacturor for help - hoping they had a 'template' for my spa... no such luck. 

Does anyone else have a Marquis Quest?  Or a 'simple' solution for getting the curved corners right?  Any Marquis dealers care to chime in with who they use for covers? 

Thanks for your help in advance.

[Spatech_tuo]

You won't necessarily get the exact same measurement using those differing methods (or even two people using the exact same method on the same cover) since its not always so evident exactly where where the straight edge meets the cover but you should get closer than your numbers indicate. Try with the square one last time and if in doubt, go with the smaller measurement and you'll be fine, it will just overhang slightly which is perfectly ok.