+lpp14 = lpp10 + [1,0] * keeper_inner_width;
+// exact x posn not very important; must extend past end of keeper
+
+lpp15 = [ lpp14[0],
+ epp0[1] - phone_rim_depth + 1/2.5 * case_th_lid ];
+// ^ beam theory says to maximise force before contact,
+// the gap below the `beam' (the lid) must be 1/3
+// the thickness (ie the lid thickness) if the beam
+// is solid, or 1/2 if it has a top and bottom only.
+// ours is mostly solid.
+
+lp_r12 = max(case_th_lid - (lpp11[1] - lpp15[1]),
+ case_th_lip);