To find the center of a circle with radius r3 that touches 2 other cirles with radii r1 & r2, find the intersection points of the existing 2 circles with radii expanded to r1+r3 & r2+ r3. Google 'intersection points of circles' to find out how. You can implement this in an Excel sheet. One of the intersection points is the one you want. The distance between the centers of adjacent circles following a spiral are exactly r1+r2 apart. The spiral is constructed in Excel (1point/1degree of the angle). Have Excel calculate the distance from a given point to all other spiral points, and find the one that matches best r1+r2. That is he center of your next circle. It appears that the match does not have to be that accurate.