Whenever I get around to adjusting the cable at the gearbox end, I remove the rear wheel, at least. Otherwise, my hands just do not fit. To adjust at the gearbox end, the cable MUST be completely free at the handlebar. If, after carefully re-routing the cable again, you are finding it too short still, you can remove the lever from its perch & slot the nipple into its recess & re-attach. (I will often do this - don't drop the top-hat spacer). Or you could loosen the perch at the handlebar clamp (2x8mm bolts) to give you an extra inch or so. Then put it all back & tighten it up before winding the adjuster out for the final adjustment. Should be about 2mm free play - the width of a coin. Then the clutch should engage & release when the lever is further out from the handlebar. Like most things, it gets easier the more you do it. (By which time, like me, you might have buggered up a few of the component bits every now & again - have a drink & get in some spares for next time).