[fulvioterzapi]

Errors do not sum like regular numbers. You want to take the square root of the sum of the squares of the errors.

sqrt(6.4^2 + 6.9^2) ≈ 9.4

You can have a look here: http://ipl.physics.harvard.edu/wp-uploads/2013/03/PS3_Error_...

[stocknoob]

That PDF is great, thanks for sharing.