War sez:
Hit however you want, but move fast, aim between the joints you want to dm, and ghost into uke a bit. If you ghost into him, you contact more joints at once and you spread them apart from each other, so the force keeping joints together damages them more and makes dms easier.
I find the best speed for a boom to be 35-55. I've done one at 60, but it was all luck. Too fast and you ghost all the way through, too slow and you don't get into uke far enough.

well counter weight manipulation helps. lets say you have a lowered arm. .and you're kicking someone raise your arm and you will gain a few more newtons to your kick

To the above, believe it or not, many joint changes that would usually seem insignificant can often change the out come of an impact. I was once lookong at a replay with nuthug and saw that he turned a 3dm boom into a 4dm boom just by relaxing all, and changing 2 joints. So my suggestion would be to experiment with a couple of joint variations when making a replay to see what gives you the best result.

And yes, a too fast of a speed can often move a body part so fast that it passes through a joint in less than one frame which does not allow for impact dismemberment to occur whilst keeping impact damage true to it's origional design. Nuthug has a replay where he shows three ghost hits in a row at speeds of 60+ aswell. Ghost hits occur most often in the elbow, wrist, and knee. I personally have never seen a ghost hit that went straight threw the chest.

I did it, i rotate the chest and contract the glutes to make enough momentums and im aiming in beetween the pecs, i followed Notice how my foot is inside Ukes torso, but it hasn't actually collided with it yet. Now look at the result a few frames later... and i got it
should i post the replay?

What distance do I set myself at?

If you want to kick him, probably just a bit shorter than your whole leg. If you want to punch him, then you can probably do it just fine if it's not too close or too far away.

If you're talking about engage distance, it doesn't make that much of a difference. If you feel like you're too close, you can push Uke away before hitting him, and if he's too far away you can just get to him and do some cool acrobatics on your way.

Is the good like a boom hit?

Sorry to say, Eruki, but I didn't see a boom hit in that replay.
A boom has to be at least 3 DM's im 1-maybe 2 frames.

Example of one of mine:

Is this a good boomhit? I edited this replay like a million times to get it.

Yep, that third hit was a boom alright.
Good job.