A good idea would be to fix the bots as they grow. Then, always make the bots reproduce upwards, let the tie swing down, and fix the bot as soon as its on the radius of the centre of curvature.
cond
*.cx *.xpos sub *.cy *.ypos sub pyth .radius %=
start
.fixpos inc
sop
Where cx is the x coord of COC, cy is the y-coord, and .radius is the size of the cap. %= will check accuracy to 10% of the expected value.