How likely is that a blood clot on a leg causes a stroke?

Rare. A blood clot affecting a leg rarely causes a stroke. It tends to occur when there is a small hole in the heart, such as a patent foramen ovale, this is something people are born with. It also tends to occur when there is a somewhat large clot in the leg, such as in the femoral vein. Blood clots in leg veins are not a major cause of stroke, like smoking, diabetes and high blood pressure are.
Quite unlikely. Blood clots in the leg(also known as DVTs) when the move, move towards the heart and tend to settle in the lungs, causing a pulmonary embolus(lung blood clot). It usually takes a transition through an uncommon hole in the heart for a DVT to spread to the brain to cause a stroke. Not impossible but very uncommon.