The knuckle is the transition zone between the table top and the landing zone. You want
to clear the knuckle so that you land in the landing zone.
You are doing what 90% of us all tend to do and that is sometime put the cart before the horse. Bottom line on this issue is if you are asking then you have taken a little bit bigger step than you should have on this. Let`s step it back to smaller features and go faster and bigger on those before stepping up to the large one.
Landing flat (on the table top) because you are coming up short and not clearing the knuckle is a real knee killer and you will regret this later on. Pick features that you can always count on clearing the table and landing in the zone. A word of caution though, you can over do this and start over shooting the landing zone by going too big and this is what injures and sometimes kills riders. Keep it in the zone!
Now for the tech...
if your landings are sketchy, then some other part of the maneuver is flawed. All freestyle can broken down into the ATML model. Approach, Takeoff, Maneuver, Landing. If any or all of the first three are off, the landing is screwed.
Above all, it is my personal opinion that a stabilized approach to any feature is the most important, followed by a correct and stable take off. This means having your speed and trajectory dialed in long before you hit the takeoff ramp. You have your body set up correctly for the maneuver and are relatively "quiet".
Your takeoff should be an extension of your approach; a following through of what you started. Remaining stable in the takeoff is key. Many riders totally loose it in the take off and flail all over the place.
Your maneuver right now should just be a good, clean straight air with or without a simple grab like an Indy or Method. Paying attention to what you do with your body in the air is crucial. It is easy to shift your weight and get off balance in the air which will kill your landing.
If everything else went well, the landing should be almost effortless. All you need to do is position your body appropriately to the speed you are traveling and the slope you are landing upon. Having good edging skills are necessary for landings because you can`'t and usually seldom land perfectly flat based so you need to be able to precisely control edging to not "crash land".
Being able to do basic carving is actually a valuable skill to have when you ride the park as well.
If you really can`t carve yet, let`s work on that more and push the jumping back a bit. If you truly want some help with carving, just ask and I as well as others will be happy to give you some pointers and some edging drills to start working on as soon as your season gets started....