diff --git a/solidify/face.h b/solidify/face.h
index ab6c5cf..266b1d2 100644
--- a/solidify/face.h
+++ b/solidify/face.h
@@ -36,27 +36,33 @@ static inline double face_z0(const struct face *f, int x, int y)
 }
 
 
+static inline double z0_scale(const struct face *f)
+{
+	return f->z_step/f->x_step;
+}
+
+
 static inline double fx_to_angle(const struct face *f)
 {
-	return atan(f->fx)/M_PI*180;
+	return atan(f->fx*z0_scale(f))/M_PI*180;
 }
 
 
 static inline double fy_to_angle(const struct face *f)
 {
-	return atan(f->fy)/M_PI*180;
+	return atan(f->fy*z0_scale(f))/M_PI*180;
 }
 
 
 static inline void fx_from_angle(struct face *f, double a)
 {
-	f->fx = tan(a/180*M_PI);
+	f->fx = tan(a/180*M_PI)/z0_scale(f);
 }
 
 
 static inline void fy_from_angle(struct face *f, double a)
 {
-	f->fy = tan(a/180*M_PI);
+	f->fy = tan(a/180*M_PI)/z0_scale(f);
 }
 
 
diff --git a/solidify/overlap.c b/solidify/overlap.c
index 96ff593..22fab6e 100644
--- a/solidify/overlap.c
+++ b/solidify/overlap.c
@@ -158,6 +158,7 @@ static void merge_matrix(struct matrix *m, const struct solid *s,
 	double tm[2][2], tm2[2][2];
 	double tv[2];
 	double f_x, f_y;
+	double z0s2 = z0_scale(f)*z0_scale(f);
 
 	/*
 	 * Finally, we convert to model matrix coordinates.
@@ -180,8 +181,8 @@ static void merge_matrix(struct matrix *m, const struct solid *s,
 	 * cos a = sqrt(1/(f^2+1))
 	 */
 
-	f_x = sqrt(f->fx*f->fx+1);
-	f_y = sqrt(f->fy*f->fy+1);
+	f_x = sqrt(f->fx*f->fx*z0s2+1);
+	f_y = sqrt(f->fy*f->fy*z0s2+1);
 
 	m->a[0][0] *= f_x;
 	m->a[0][1] *= f_x;