Fix and combine JD condition

Marlin/src/module/planner.cpp

@@ -2306,12 +2306,8 @@ bool Planner::_populate_block(block_t * const block, bool split_move,
 
         vmax_junction_sqr = junction_acceleration * junction_deviation_mm * sin_theta_d2 / (1.0f - sin_theta_d2);
 
-        if (block->millimeters < 1) {
-          const float neg = junction_cos_theta < 0 ? -1 : 1,
-                      t = neg * junction_cos_theta;
-
-          // If angle is greater than 135 degrees (octagon), find speed for approximate arc
-          if (t > 0.7071067812f) {
+        // For small moves with >135° junction (octagon) find speed for approximate arc
+        if (block->millimeters < 1 && junction_cos_theta < -0.7071067812f) {
 
           #if ENABLED(JD_USE_MATH_ACOS)
 
@@ -2358,10 +2354,13 @@ bool Planner::_populate_block(block_t * const block, bool split_move,
               4.85301876f, 6.77019119f, 9.50873947f, 13.4009094f, 18.9188652f,
               26.7320709f, 37.7884521f, 53.4292908f, 75.5522461f,  0.0f };
 
+            const float neg = junction_cos_theta < 0 ? -1 : 1,
+                        t = neg * junction_cos_theta;
+
             const int16_t idx = (t == 0.0f) ? 0 : __builtin_clz(uint16_t((1.0f - t) * jd_lut_tll)) - jd_lut_tll0;
 
             float junction_theta = t * pgm_read_float(&jd_lut_k[idx]) + pgm_read_float(&jd_lut_b[idx]);
-              if (neg > 0) junction_theta = RADIANS(180) - junction_theta;
+            if (neg > 0) junction_theta = RADIANS(180) - junction_theta; // acos(-t)
 
           #else
 
@@ -2371,7 +2370,9 @@ bool Planner::_populate_block(block_t * const block, bool split_move,
             //  acos( t) = pi / 2 - asin(x)
             //  acos(-t) = pi - acos(t) ... pi / 2 + asin(x)
 
-              const float asinx =       0.032843707f
+            const float neg = junction_cos_theta < 0 ? -1 : 1,
+                        t = neg * junction_cos_theta,
+                        asinx =       0.032843707f
                               + t * (-1.451838349f
                               + t * ( 29.66153956f
                               + t * (-131.1123477f
@@ -2388,7 +2389,6 @@ bool Planner::_populate_block(block_t * const block, bool split_move,
           NOMORE(vmax_junction_sqr, limit_sqr);
         }
       }
-      }
 
       // Get the lowest speed
       vmax_junction_sqr = _MIN(vmax_junction_sqr, block->nominal_speed_sqr, previous_nominal_speed_sqr);