diff --git a/src/bin/flat.rs b/src/bin/flat.rs
index 6f7a7ae..692e708 100644
--- a/src/bin/flat.rs
+++ b/src/bin/flat.rs
@@ -637,7 +637,7 @@ mod riemann {
 		}
 
 		fn part_derivs_at(&self, pos: Vec2) -> Tens2 {
-			part_deriv(|p| self.at(p), pos, 1.0e-3)
+			part_deriv(|p| self.at(p), pos, 1.0 / 1024.0) // division by such eps is exact which is good for overall precision
 		}
 
 		fn vec_length_at(&self, at: Vec2, v: Vec2) -> f32 {