Hi som, if you're sure you cut accurately, it sounds like your angles are wrong. You have to cut equilateral, 60° triangles, that way 6 of them put together will total 360°. Hang on, I'll post a pic of my ruler...
BTW this is my old triangle with the tips cut off (not ideal!) I now have a new one with the tips but the angles are the same. Anyone who is buying one, get one with tips, the tips are important when making these as they help you line everything up! :-D
Here's my triangle with my 6.5" square laying alongside it.
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