Circle fit in pointcloud, stl

Hi,

for a project I need to fit a circle in an pointcloud in a certain hight of a 3d stl-plot. I already get the circle of the whole thing but not in the height i need to know it (-263,6365).

stlData=stlread("24Lcav.stl");

X=stlData.Points;

x=X(:,1);

y=X(:,2);

z=X(:,3);

x=x(:); y=y(:);

a=[x y ones(size(x))]\[-(x.^2+y.^2)];

xc = -.5*a(1);

yc = -.5*a(2);

R = sqrt((a(1)^2+a(2)^2)/4-a(3))

When I set x=x(-263,6365) matlab sais it has to be positiv.

I hope you can help me.