Throttle open all the way and kickdown lever full back.
The above usually results in late part-throttle shifts.
The problem with the generic cable setups is the different lengths of the transmission lever.
You need to measure the full travel of the carb lever where the cable attaches and then measure the full travel of the transmission lever where the cable attaches; it usually won't be the same so you need to drill a new hole in the transmission lever so that the full travel of both levers is the same. Also, there should be a spring pulling the transmission lever forward.
It isn't necessary for the trans lever to be full back at WOT but be sure that WOT under the hood is the same as WOT at the pedal...misadjusted throttle linkage will prevent the carb from going wide open when the pedal is matted and that, in turn, will prevent the trans lever from going back enough to provide kickdown.
Remove the cable from the carb and measure how far the hole in the throttle lever moves from idle to WOT. Then measure how far the hole in the trans lever moves from full forward to full back. The movement of the trans lever will likely be more than the carb lever so find the spot on the trans lever that matches the travel of the carb lever and drill there.
It's unlikely that the trans lever will return to the full forward position without a spring assist.
