http://www.conitec.net/beta/aclamp.htm
clamp(var x,var a,var b);


Code:
var limit =0 ;

while(x)
{
..... 

 limit=clamp(camera.tilt,0,90); //min max
 ent_bonerotate(my, "Bone", vector(0,limit,0));

..........
}


Beating a dead horse...