sorry if this is too long...
i never got the parallel part (who even goes parallel on a back crook?) but everything else mentioned here works wonders.
if i go at a very slight angle so i don't have to twist or shifty into it it's much easier to align your head over chest, chest over hips, hips over front ankle completely on top of the ledge and use your hips to counter balance the pinch.
focus on popping above the ledge and coming down, not bashing into it like a back crook.
hang your toes off like a heelflip and focus on a proper ollie and pinch it with the weight on your heel or arch of your foot exactly on the corner/top of the ledge.
if your toe drags you can stick your butt out to take the weight off and counter balance.
biggest problem i run into is the angles: going too parallel and getting into that front nose/front crook thing where your back foot comes off and your front foot just pushes it forward, or going too much of an angle and not pinching/getting into a nosegrind sketchy crook but you usually know exactly what went wrong when that happens. i feel like it's less scary on a rail to go at more of an angle and just commit to smashing your front foot on the rail while leaning forward. as for going further... getting up on top and using your butt to counter balance was a big a-ha moment for me, along with not being afraid to just wax the ledge where the nose is pinching up top. to get off i just practised standing in it and hopping off the ledge a million times. don't listen to people who say learn to go fakie since it's easier, it will ruin your technique.