66 dart LED prototype working for now

You've created a system of four stop/tail lamps (left and right, inboard and outboard). The intensity and ratio requirements apply to each lamp, that is, inboard and outboard are treated as separate lamps, so make your test on only one lamp at a time.

Test points are specified in degrees Up or Down from Horizontal, and Left or Rright from Vertical, plotted on cartesian coordinates as you can see in the grid I posted above. So for example, a test point (H,V) refers to the intersection of the vertical and horizontal axes, that is straight back from the optical center of the lamp. (H, 5L) means zero degrees up or down from the horizontal axis, and 5 degrees to the left of the vertical axis. (5U, 10R) means 5 degrees up from the horizontal axis, 10 degrees to the right of the vertical axis. And so on.

Get a sturdy flat board and mark it up as a square grid with a ruler. Make your grid lines just a hair under 10.5" apart (the actual figure is 10.47"). Make sure the board is big enough so that the center of the grid can be aligned with the optical center of the lamp being tested. With this board held or mounted vertically (on a wall makes life easiest) exactly 10 feet away from the lamp, each grid line intersects at 5-degree intervals. So you can easily and repeatably measure at the relevant points to get the performance dialled in just right.

Mount the lamp in the car so it will be at the correct attitude (tilt angles, etc.). Align the center of your grid (which you should mark conspicuously so you can easily keep track of it as you do the test) carefully with the optical center of the lamp. This is easy enough to do vertically; measure the height from ground to optical center of lamp and then measure up that same distance from ground to grid center. It's important that the ground be at the same level between the lamp and the test board. It's harder to align horizontally, but an adjacent wall can help here, too, as long as it's straight: measure over from the wall to the optical center of the lamp, then shift the grid horizontally til its center is that same distance from the wall.

Use a good luxmeter (or the luxmeter mode of a multipurpose light meter). Wait til it's well and truly dark out. Turn off all the extraneous lights you can (house, garage, etc.), then take and record a baseline reading because there'll still be some stray light around. Now turn on the light to test in the dim (tail) mode and take readings at every grid intersection point in the range 10U to 10D, 20L to 20R. Try to take multiple readings at each point as a sort of on-the-fly reality check. Write down each reading. Now switch the lamp to bright (stop) mode and take readings again throughout the same grid. While you're at it, double-check for intensity maintenance with prolonged operation: take a reading at (H,V) immediately after switching to bright mode, then take another reading after 10 minutes' steady operation, and another reading after 30 minutes.

Now go to a desk and do some math. Subtract your baseline "all lights off" reading from each of the grid readings you took. Now multiply all your grid readings by 9.29 to turn the lux values into candela values; these can be used to check against the applicable technical standards (send me a PM and I'll send them to you) to make sure you're within the required intensity range for the lighting functions you're doing. Basics: the maximum allowable intensity from the stop lamp function is 300 candela; for the tail lamp function the maximum is 18 candela. The minimum for both functions varies by test point.

The bright:dim intensity ratio must be at least 5:1 at points (H,V); (H,5L); (H,5R), and (5U,V). At all other points, the ratio must be at least 3:1.

You may want to complete the LED upgrade with a CHMSL -- if you decide to make your own, the CHMSL function has its own requirements; I can send those to you, too.

If you can wring at least 500cd at (H,V) out of the fronts in bright mode, while keeping the emitters well enough heat sunk to keep intensity above 500cd with prolonged operation, they qualify to be operated as DRLs.

You may have started out just wanting to get the whole rear cluster to light up for brake/turn mode, but it looks like you're going to wind up with a system that is a real safety performance improvement over original. If you've read this far, let me say I appreciate and respect how you're considering the importance of the safety aspects of the kind of upgrade you're working on. Some others have been dismissive and less thoughtful.

(Also, I like that you're using a resistive control circuit rather than PWM. It will mean your lights won't produce that irritating "bead effect" visible to observers whose gaze scans across your lamps when they're operating in dim mode.)