You can adjust the one thats already on the car . Granted you cant do it from inside the cockpit , but its rare you need to constantly change the effort going to the rear wheels .
I wound up the rear effort to 50% above standard from 80kgs to 120kgs . If your friendly with a local mot place they can measure the amount of braking force going to the wheels on their rollers . I wouldnt recommend to overdo the amount of rear effort , but I like the rear-end to swing around a bit to aid turn-in.