The fuel leak procedure does not tell you to calculate leak rate. There is a difference between leak rate and depletion rate. In your case fuel leak is confirmed from left side. So first thing as per procedure is LAND ASAP. To do that consider left side fuel as lost and select suitable diversion and head towards it. Only thing left to find out is whether it is from left tank or left engine. For that apply the procedure from ONE INNER TANK DEPLETES FASTER THAN THE BY AT LEAST 300KGS IN LESS THAN 30 MINUTES. It asks you to shut down the engine and if the left tank quantity remains static then it is from engine and has stopped. So the fuel can be balanced and used. If quantity keeps decreasing then it is from the tank so restart the engine but do not balance. Let the engine run till it flames out.