Unfortunately, double is simply a synonym for float on the Arduino, and a quick look online showed no library for higher precision floating point (but it was a quick search - if anyone finds one, I too would like to know).

The float is a 32 bit variable, which ends up being about 6-7 digits of precison, and would explain the problem here:

http://www.arduino.cc/en/Reference/FloatFor example, even your value of inc (0.1) won't store precisely, so you'll have problems.

One thought - convert these to longs:

long c0offset = -60;

long c1offset = 0;

long c2offset = 60;

long d01 = c1offset - c0offset;

long d02 = c2offset - c0offset;

Since they all seem to be integer anyways, and then change the formula:

x01 = ((sq(d01) - sq(Rcircle1) + sq(Rcircle0)) / (2.0 * d01)) + c0offset;

to

long temp1=2*d01;

long temp2=d01*d01;

x01 = ((temp2 - sq(Rcircle1) + sq(Rcircle0)) / temp ) + c0offset;

It won't do much, but you'll save two floating point multiplies, and resultant loss of precision.