diff --git a/boxtree/traversal.py b/boxtree/traversal.py
index 6960e45c79cb9343ddc5711ad82ec16c164eddeb..90510674825b62981aa1c3d21f7d7493ab51b99d 100644
--- a/boxtree/traversal.py
+++ b/boxtree/traversal.py
@@ -794,8 +794,8 @@ void generate(LIST_ARG_DECL USER_ARG_DECL box_id_t target_box_number)
                             coord_t l_2_box_dist =
                                 sqrt(l_2_squared_center_dist)
                                 - sqrt((coord_t) (${dimensions}))
-                                    * tgt_stickout_l_inf_rad
-                                - source_l_inf_rad;
+                                  * (tgt_stickout_l_inf_rad
+                                    + source_l_inf_rad);
 
                             meets_sep_crit = l_2_box_dist >=
                                 (2 - 8 * COORD_T_MACH_EPS) * source_l_inf_rad;
@@ -921,6 +921,24 @@ inline bool meets_sep_bigger_criterion(
     coord_vec_t source_center, int source_level,
     coord_t stick_out_factor)
 {
+    <%
+        assert not sources_have_extent
+    %>
+
+    // What we are interested in ensuring is that
+
+    // (*)
+    // d_2(src_box, tgt_center)
+    //     >= 3 * (radius of tgt box potentially
+    //                   including stick-out)
+
+    // (because convergence factors are in l^2,
+    // irrespective of how we measure)
+
+    // Since d_2(a, b) >= d_inf(a, b), ensuring that
+    // (*) holds with d_inf implies that it also holds
+    // with d_2.
+
     coord_t target_rad = LEVEL_TO_RAD(target_level);
     coord_t source_rad = LEVEL_TO_RAD(source_level);
     coord_t max_allowed_center_l_inf_dist = (