Are you trying to work up from lat pull downs to pull ups? To some degree, lat pull downs and regular pull ups work the same muscles, but there are a couple of big differences between them. Check out this Pull ups vs Lat Pull Downs video and find out if that’s a good idea or not.
The fact is the lat pull doesn’t transfer too well to the pullup because the body isn’t being stabilized the same way. If you want to increase this transfer I would encourage you to not lock yourself in with your legs under the pads but to keep your body free.
This is one method I have been using when working towards the one arm chinnup using a lat pull down machine.
Instead of a lat pull down if you’re trying to build up to your first pullup try something like jumping up and doing slow control negatives.
Visit this link to discover a way to work your way up to that first pull-up, how to start training for one-arm pull-up and techniques to raise pull-up repetitions.