Carburettor Throttle Valve Adjustment

I am about to try to adjust my throttle valves on my carbs using the wire-drop technique, however, it looks like the throttle valve is lower at the back than the front. On carb #3 here you can just about see daylight:

Looking at this photo, you can see a very big gap at the front curved edge of the throttle valve, but the rear edge is a lot closer to the carb body.

My question is—should I be clamping the wire in between the back edge of the throttle valve and the carb body, and ignoring the front edge?

Engine side. This is what the 1981 KZ650 FSM shows.

I disconnected the throttle spring so that the slide notches wouldn't get buggered. my throttle slides have little cutouts or notches. Also, I set #1 to about 1.0 mm (it doesn't have to be perfect), then used 2 identical drill bits to set the other 3 >> put one drill bit in the #1 carb & leave it there, then use the second drill bit to set the others. easy. Make sure the big throttle stop thumb screw is set at about the midway point, so that you'll have some adjustment either way when done. Some guys do the slide adjustment, then find that the throttle screw can't be turned out or in any further.

You can do it on either front or back depending on the width of wire used or just use your eye to judge the equal daylight showing under each slide.
You can also level the front curved edge against the top of the carb bore (venturi) to achieve the same result and having the throttle return spring removed makes it easier if doing it that way.
Bench syncing only roughly gets the carbs in the ballpark area for balance though and once on the bike you will still need to vacuum balance them to ensure each cylinder is doing equal work.

The wire-drop technique is for the output port of the carbs not the Input with the slider cutoff.

You say on carb 3 the throttle valve/slide is lower at the back than the front, do you have pictures of the back and front of carb 3, have you got the slide in the right way round?