diff --git a/src/scgenerator/evaluator.py b/src/scgenerator/evaluator.py
index 7980b49..339647f 100644
--- a/src/scgenerator/evaluator.py
+++ b/src/scgenerator/evaluator.py
@@ -509,25 +509,11 @@ default_rules: list[Rule] = [
     Rule("effective_area", fiber.effective_area_hasan, conditions=dict(model="hasan")),
     Rule("effective_area", fiber.effective_area_from_gamma, priorities=-1),
     Rule("effective_area", fiber.effective_area_marcatili, priorities=-2),
-    Rule("effective_area_arr", fiber.effective_area_from_V, ["core_radius", "V_eff_arr"]),
+    Rule("effective_area", fiber.effective_area_from_pitch),
+    Rule("effective_area_arr", fiber.effective_area_from_V, ["core_radius", "V_eff"]),
     Rule("effective_area_arr", fiber.load_custom_effective_area),
-    Rule(
-        "V_eff",
-        fiber.V_parameter_koshiba,
-        ["wavelength", "pcf_pitch", "pcf_pitch_ratio"],
-        conditions=dict(model="pcf"),
-    ),
-    Rule(
-        "V_eff",
-        fiber.V_eff_step_index,
-        ["l", "wavelength", "core_radius", "numerical_aperture"],
-    ),
-    Rule("V_eff_arr", fiber.V_parameter_koshiba, conditions=dict(model="pcf")),
-    Rule(
-        "V_eff_arr",
-        fiber.V_eff_step_index,
-        ["l", "core_radius", "numerical_aperture", "wavelength_window"],
-    ),
+    Rule("V_eff", fiber.V_parameter_koshiba, conditions=dict(model="pcf")),
+    Rule("V_eff", fiber.V_eff_step_index),
     Rule("n2", materials.gas_n2),
     Rule("n2", lambda: 2.2e-20, priorities=-1),
     Rule("gamma", lambda gamma_arr: gamma_arr[0], priorities=-1),
@@ -557,6 +543,7 @@ default_rules: list[Rule] = [
 envelope_rules = default_rules + [
     # Grid
     Rule(["w_c", "w", "w_order"], math.build_envelope_w_grid),
+    Rule("dt", math._dt_from_wl_window),
     # Pulse
     Rule("spectrum_factor", pulse.spectrum_factor_envelope, priorities=-1),
     Rule("pre_field_0", pulse.initial_field_envelope, priorities=1),
diff --git a/src/scgenerator/math.py b/src/scgenerator/math.py
index 11d1bbe..bda2c70 100644
--- a/src/scgenerator/math.py
+++ b/src/scgenerator/math.py
@@ -171,24 +171,24 @@ def tspace(time_window=None, t_num=None, dt=None):
 
     Parameters
     ----------
-        time_window : float
-            total time spanned
-        t_num : int
-            number of points
-        dt : float
-            time resolution
+    time_window : float
+        total time spanned
+    t_num : int
+        number of points
+    dt : float
+        time resolution
 
-        at least 2 arguments must be given. They are prioritize as such
-        t_num > time_window > dt
+    at least 2 arguments must be given. They are prioritize as such
+    t_num > time_window > dt
 
     Returns
     -------
-        t : array
-            a linearily spaced time array
+    t : array
+        a linearily spaced time array
     Raises
     ------
-        TypeError
-            missing at least 1 argument
+    TypeError
+        missing at least 1 argument
     """
     if t_num is not None:
         if isinstance(time_window, (float, int)):
@@ -203,6 +203,14 @@ def tspace(time_window=None, t_num=None, dt=None):
         raise TypeError("not enough parameter to determine time vector")
 
 
+def dt_from_min_wl(wl_min: float, wavelength: float) -> float:
+    return 0.5 * 1 / c * 1 / (1 / wl_min - 1 / wavelength)
+
+
+def _dt_from_wl_window(wavelength_window: tuple[float, float], wavelength: float) -> float:
+    return dt_from_min_wl(wavelength_window[0], wavelength)
+
+
 def build_envelope_w_grid(t: np.ndarray, w0: float) -> tuple[np.ndarray, np.ndarray, np.ndarray]:
     """
     convenience function to
diff --git a/src/scgenerator/physics/fiber.py b/src/scgenerator/physics/fiber.py
index e7aeb8d..1cda9a2 100644
--- a/src/scgenerator/physics/fiber.py
+++ b/src/scgenerator/physics/fiber.py
@@ -422,7 +422,7 @@ def V_eff_step_index(
     return pi2cn / l
 
 
-def V_parameter_koshiba(l: np.ndarray, pcf_pitch: float, pcf_pitch_ratio: float) -> float:
+def V_parameter_koshiba(l: np.ndarray, pcf_pitch: float, pcf_pitch_ratio: float) -> np.ndarray:
     """
     returns the V parameter according to Koshiba2004
 
@@ -445,7 +445,7 @@ def V_parameter_koshiba(l: np.ndarray, pcf_pitch: float, pcf_pitch_ratio: float)
     """
     ratio_l = l / pcf_pitch
     n_co = 1.45
-    a_eff = pcf_pitch / np.sqrt(3)
+    a_eff = effective_area_from_pitch(pcf_pitch)
     pi2a = pipi * a_eff
     A, B = saitoh_paramters(pcf_pitch_ratio)
 
@@ -457,6 +457,10 @@ def V_parameter_koshiba(l: np.ndarray, pcf_pitch: float, pcf_pitch_ratio: float)
     return V_eff
 
 
+def effective_area_from_pitch(pcf_pitch: float):
+    return pcf_pitch / np.sqrt(3)
+
+
 def effective_area_from_V(core_radius: float, V_eff: T) -> T:
     """
     According to Marcuse1978
diff --git a/tests/test_evaluator.py b/tests/test_evaluator.py
index 0397a11..d665f5e 100644
--- a/tests/test_evaluator.py
+++ b/tests/test_evaluator.py
@@ -36,7 +36,7 @@ def test_simple():
     assert evaluator.compute("w0") == pytest.approx(units.nm(800))
 
 
-def test_default_args():
+def test_default_args_simple():
     def some_function(a: int, b: int, c: int = 5):
         return a + b + c
 
@@ -51,3 +51,19 @@ def test_default_args():
     evaluator.set(c=10)
     assert evaluator.compute("c") == 10
     assert evaluator.compute("d") == 14
+
+
+def test_default_args_real():
+    evaluator = Evaluator.default()
+    evaluator.set(
+        wavelength=1050e-9,
+        peak_power=5000,
+        width=1500e-15,
+        wavelength_window=(800e-9, 1500e-9),
+        t_num=2048,
+    )
+
+    assert evaluator.compute("dt") == pytest.approx(math.dt_from_min_wl(800e-9, 1050e-9), abs=0)
+    assert evaluator.compute("t") == pytest.approx(
+        math.tspace(t_num=2048, dt=evaluator.compute("dt")), abs=0
+    )