Leverage and positioning of ALL the levers is critical.
The most significant one is the Handle bar lever. The Steel hand brake lever pivot points are not optimal, and has excessive travel, so the location of the cable nipple in relation to the hand lever pivot point is crucial. I set the cable free play so that the position of the cable upper nipple is JUST sweeping past 90 degree of the fulcrum point.
It means that there seems to be excessive free play in the hand lever, but doing so optimise hand pull leverage.
(I occasionally take up the free play by setting a spare rear chain master-link interposed between the Perch and the Blade)
BOTH cables MUST have the same degree of free play, so that the wiffle beam is square to the cables and the handle bar perch.
Both levers at the brake plate MUST be set identically, so that when fully applied, the levers are approaching 90 deg relative to an imaginary line passing through the brake shoe pivot.
Leave the cam pivot free to just slide. It is important that the DOUBLE helix spring washers that secure the cam plates nuts have not been replaced with 'common' spring washers? They are an important feature of the working design. If in doubt, Dot Punch the nuts after the cams have been set.
Keep in mind that the double-pull set up is always going to have a heavy pull, so I set it so that the blade NEARLY comes all the way back to the bar.