tab_h = tot_height - ap_height;
-real_ap_sz = [ ap_width, ap_height ] - app_slop * [ 1,1 ];
-real_main_sz = real_ap_sz + wall_th * [ 2,1 ];
+real_main_sz = [ ap_width, ap_height ] - app_slop * [ 1,1 ];
real_all_sz = real_main_sz + tab_h * [0,1];
+real_inner_sz = real_main_sz - wall_th * [ 2,1 ];
screw_pos = real_all_sz - (screw_ctr_to_tr - 0.5 * app_slop * [1,1]);