From where the front lines meet... Make it the length of your kite - that allows for when it goes Nuclear, and you want your kite to lay down & die.
It can be shorter but choose carefully the conditions your gonna use it for.
Just because a kite flags to one line it doesnt always mean your gonna stop, when its nuclear + ive been barefoot sand skiing - pulled out of the water, towed by a flagged kite
... Untill 2 mates jumped on it
Better safe than sorry