No, Cabrinha has no bridles for its BT.
The idea is good. Copy the line lengh from a wipika 11.8 or 16.4....Anyway bridle size doens't matter too much. Your 15.5 has narrow tips, perhaps close to the size of your 8.4. Tip size is a important variable in bridle geometrie.
Of course the third line lengh has to be enough to not get in tension when you steer the kite.
Other good option to BT is put the tip battens as the 2002 model. It helps a lot.
Last year many had made this with good results. If you like the light forces on the bar, the go with the bridle, or both.