Calais light has the characteristic Fl(4)15s59m22M which interpreted means: 4 flashes repeated every 15 seconds. 59m above sea level with a range of 22 nautical miles. Cap Gris Nez is Fl 5s72m29M or one flash every 5 seconds at a height of 72 m visible for 29 nautical miles. So, on a clear night you should be able to just see both. In theory a vessel without modern navigational instruments can travel from France to England using the Cap Gris Nez to Folkstone passage just by observing the lights. BTW, Folkstone is Fl(2)10s22M.