Actually the lowest wind limit will depend on your kite (weight and size), hyfrofoil area and skills. So your limits are not the same as for each kind people here, and is very personal. Hence this even add on the complexity of describing their own limit. Forget speed meters unfortunatelly ...
Personally i use 3 things in on shore conditions (experienced light weight rider with the optimum foilkite) :
- the feeling of wind on my face: if i can guess the direction of the wind with eyes closed, it is ok with me.
- the water surface in open sea: small local glassy areas are still acceptable but it is my limit, but being psychologocally ready to swimback . Without glassy part like oil, i can ride with very few risks.x
- flags in exposed area (but can change with flag weight): about 1 second frequency and 45 degree angle is my limit.
BUT : since you should assess your own limit with your kite, best is to try with your kite everytime, which is anyway a good exercice to know your kite and spot by heart (including waterstarts laid on the beach), and you will build your proper references.
Then ... enjoy your unforgettable succees in very light winds, riding 3 times faster than no-wind