Would you believe that according to Airbus, high flex temperatures actually consumes more fuel overall on the climb out.. About 4 kgs for the A320.
Anyways ..... I know I use a timing "in principle" method. On the A320 it takes about 5 seconds to accelerate from 80kts to 100kts at a high flex setting. This coincides with the reduction of forward pressure on the side stick.
Food for thought.