The length and angle is a compromise, and about more than just the trade between force and travel. Let's use the Mafac Racer brakes above as an example. If you have a very short cable (assuming when the pads contact the rims), pulling up gives a very high straddle cable tension, but not necessarily harder braking, becaused what also matters is the angle of that cable to a line from the arm end to the arm pivot; With a very short cable, it's not pulling at 90 degrees to that line, and 90 degrees is the best leverage angle on the arm. However, shortening the straddle cable to get more force, may have more positive effect, than the negative effect of the pull angle on the arm. This could all be trigged out if you desire, by sketching it all out, with the brake arms at a position where the pads contact the rim, and start with the straddle cable angle and its tension for a given vertical pull, then see how that subsequent pull angle on the arm calculates out. Then look at extremes, shorter and longer straddle cable. Or, you could experiment on the bike.