I don't see any issue with calculating 8x8. Putting a tangent on the blue 8 results in an intersection at the red "6.4".

Probably you are alluding to the fact that numbers are treated equal if they are only different by a factor 10^x (such as ..., 0.064, 0.64, 6.4, 64, 640, 6400, ...). In that case, see the comment by David Stay and the reply he got:

https://cp4space.wordpress.com/2012/08/29/elliptic-curve-cal...