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

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.

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.