diff --git a/csiborgtools/flow/flow_model.py b/csiborgtools/flow/flow_model.py
index 85e1d46..210e6cb 100644
--- a/csiborgtools/flow/flow_model.py
+++ b/csiborgtools/flow/flow_model.py
@@ -378,7 +378,7 @@ def sample_simple(e_mu_min, e_mu_max, dmu_min, dmu_max, alpha_min, alpha_max,
 def sample_calibration(Vext_min, Vext_max, Vmono_min, Vmono_max, beta_min,
                        beta_max, sigma_v_min, sigma_v_max, h_min, h_max,
                        rLG_min, rLG_max, no_Vext, sample_Vmono, sample_beta,
-                       sample_h, sample_rLG):
+                       sample_h, sample_rLG, sample_Vmag_vax):
     """Sample the flow calibration."""
     sigma_v = sample("sigma_v", Uniform(sigma_v_min, sigma_v_max))
 
@@ -387,10 +387,18 @@ def sample_calibration(Vext_min, Vext_max, Vmono_min, Vmono_max, beta_min,
     else:
         beta = 1.0
 
+    if not no_Vext and sample_Vmag_vax:
+        raise RuntimeError("Cannot sample Vext and Vext magnitude along the "
+                           "void axis simultaneously.")
+
     if no_Vext:
         Vext = jnp.zeros(3)
-        # 840 in the direction of (l, b) = (117, 4)
-        # Vext = jnp.asarray([338.9478154 , -11.45056064, 768.49415294])
+
+        if sample_Vmag_vax:
+            Vext_mag = sample("Vext_axis_mag", Uniform(0.0, Vext_max))
+            # In the direction lf (l, b) = (117, 4)
+            Vext = Vext_mag * jnp.asarray([0.4035093, -0.01363162, 0.91487396])
+
     else:
         Vext = sample("Vext", Uniform(Vext_min, Vext_max).expand([3]))
 
diff --git a/csiborgtools/read/paths.py b/csiborgtools/read/paths.py
index 4e012e9..9941ca8 100644
--- a/csiborgtools/read/paths.py
+++ b/csiborgtools/read/paths.py
@@ -694,7 +694,7 @@ class Paths:
                         sample_beta=False, no_Vext=None,
                         sample_Vmono=False, sample_mag_dipole=False,
                         sample_curvature=False, absolute_calibration=None,
-                        verbose_print=True):
+                        sample_Vmag_vax=False, verbose_print=True):
         """Flow validation file path."""
         if isinstance(catalogue, list) and len(catalogue) == 1:
             catalogue = catalogue[0]
@@ -710,10 +710,11 @@ class Paths:
         keys = ["smooth", "nsim", "zcmb_min", "zcmb_max", "mag_selection",
                 "sample_alpha", "sample_beta", "no_Vext", "sample_Vmono",
                 "sample_mag_dipole", "sample_curvature",
-                "absolute_calibration"]
+                "sample_Vmag_vax", "absolute_calibration"]
         values = [smooth, nsim, zcmb_min, zcmb_max, mag_selection,
                   sample_alpha, sample_beta, no_Vext, sample_Vmono,
-                  sample_mag_dipole, sample_curvature, absolute_calibration]
+                  sample_mag_dipole, sample_curvature,
+                  sample_Vmag_vax, absolute_calibration]
 
         for key, value in zip(keys, values):
 
diff --git a/scripts/flow/flow_validation.py b/scripts/flow/flow_validation.py
index 5196e0a..5371973 100644
--- a/scripts/flow/flow_validation.py
+++ b/scripts/flow/flow_validation.py
@@ -309,7 +309,8 @@ if __name__ == "__main__":
     mag_selection = None
     sample_alpha = False if "IndranilVoid_" in ARGS.simname or ARGS.simname == "no_field" else True  # noqa
     sample_beta = None
-    no_Vext = None
+    no_Vext = True
+    sample_Vmag_vax = True
     sample_Vmono = False
     sample_mag_dipole = False
     absolute_calibration = None
@@ -325,6 +326,7 @@ if __name__ == "__main__":
                     "sample_alpha": sample_alpha,
                     "sample_beta": sample_beta,
                     "no_Vext": no_Vext,
+                    "sample_Vmag_vax": sample_Vmag_vax,
                     "sample_Vmono": sample_Vmono,
                     "sample_mag_dipole": sample_mag_dipole,
                     "absolute_calibration": absolute_calibration,
@@ -377,7 +379,8 @@ if __name__ == "__main__":
                                "beta_min": -10.0, "beta_max": 10.0,
                                "sigma_v_min": 1.0, "sigma_v_max": 5000 if "IndranilVoid_" in ARGS.simname else 750.,  # noqa
                                "h_min": 0.01, "h_max": 5.0,
-                               "no_Vext": False if no_Vext is None else no_Vext,  # noqa
+                               "no_Vext": False if no_Vext is None else no_Vext,        # noqa
+                               "sample_Vmag_vax": sample_Vmag_vax,
                                "sample_Vmono": sample_Vmono,
                                "sample_beta": sample_beta,
                                "sample_h": sample_h,