If you are that handy with shop tools. You can figure out the dimensions pretty easily.
Rule#1; When the carb is at WOT, the lower lever has to be all the way back, for long tranny life. But not so far back that it interferes with the carb going WOT.
When the carb is idling, the lever can be pretty much anywhere it falls.
The slot is there to allow the carb to return to idle, no matter where the lower lever returns to. And to allow some adjustment to soften the upshift hit in day to day driving. I would make the slot about 3 inches long. I would set it up with a very light return spring on it,and drive it.After you get it where you like it; cut the unused part of the slot off and strap it so's it can't fall off....Or just leave it if it doesn't interfere with anything.
The firewall end of the slot does not HAVE to remain in contact with the carb pin.The pin can probably land anywhere, but the sooner it hits, the sooner the line pressure will begin to rise. But above all, see rule #1.