Apply code-review comments.

- Add comments to previously undocumented function.
- Add round brackets to a foruma for readability.
- Add assert to detect situations where the closed polygon loop has a strange numner of vertices.

part of CURA-9474

include/InsetOrderOptimizer.h

@@ -113,6 +113,13 @@ class InsetOrderOptimizer
     Shape retraction_region_; // After printing an outer wall, move into this region so that retractions do not leave visible blobs. Calculated lazily if needed (see
                               // retraction_region_calculated).
 
+    /*!
+     * Given a closed polygon, insert a seam point at the point where the seam should be placed.
+     * This should result in the seam-finding algorithm finding that exact point, instead of the
+     * 'best' vertex on that polygon. Under certain circumstances, the seam-placing algorithm can
+     * however still deviate from this, for example when the seam-point placed here isn't suppored
+     * by the layer below.
+     */
     void insertSeamPoint(ExtrusionLine& closed_line);
 
     /*!

src/InsetOrderOptimizer.cpp

@@ -171,6 +171,7 @@ bool InsetOrderOptimizer::addToLayer()
 void InsetOrderOptimizer::insertSeamPoint(ExtrusionLine& closed_line)
 {
     assert(closed_line.is_closed_);
+    assert(closed_line.size() >= 3);
 
     Point2LL request_point;
     switch (z_seam_config_.type_)
@@ -212,7 +213,7 @@ void InsetOrderOptimizer::insertSeamPoint(ExtrusionLine& closed_line)
     const coord_t total_dist = vSize(end_pt.p_ - start_pt.p_);
     const coord_t start_dist = vSize(closest_point - start_pt.p_);
     const coord_t end_dist = vSize(closest_point - end_pt.p_);
-    const coord_t w = end_pt.w_ * end_dist / total_dist + start_pt.w_ * start_dist / total_dist;
+    const coord_t w = (end_pt.w_ * (end_dist / total_dist)) + (start_pt.w_ * (start_dist / total_dist));
 
     closed_line.junctions_.insert(closed_line.junctions_.begin() + closest_junction_idx + 1, ExtrusionJunction(closest_point, w, start_pt.perimeter_index_));
 }