You can start with the list of triples and then find a fit with your favourite formula:
triples = {{1, 0, 0}, {1, 4, 50/3}, {1, 5, 20},
{1, 20, 50}, {1, 30, 60}, {1, 60, 75},
{10, 1, 1.25}, {10, 11, 12.25}, {10, 20, 20.25},
{10, 11, 12.25}, {10, 40, 101/3}, {10, 78, 49.75},
{10, 100, 55.93}, {10, 150, 65.56}, {10, 200, 71.74},
{10, 250, 76.04}, {40, 1, 0.26}, {40, 10, 2.51},
{40, 50, 11.39}, {40, 100, 20.45}, {40, 200, 33.95},
{40, 500, 56.24}, {40, 1000, 71.99}, {40, 1500, 79.4}};
FindFit[triples, formula, parameters, {l, v}]
I found that the following function gives a good fit:
53.5507 ArcTan[
0.557648 (1 - 0.00014188 (-10 + l) (-1 + l)) Sqrt[v/(2 + l)]]^2