Units are in mm on Mecabricks. LDU unit is incorrect for a lot of parts and that is why you cannot perfectly convert from LDD (Mecabricks) to Ldraw.
E.g. Technic bricks have the hole at a height of 5.8mm from ground on real piece. 5.8mm / 0.4 = 14.5LDU but because they don't divide the unit, they put the hole at 5.6mm (14 LDU).

I pretty much never use the Position settings in the Property panel (I need to rework that to make it more efficient anyway). When you select a position field it returns the raw float as per what javascript is calculating and it can result in numbers with a lot of 9s (you can search on google if you are interested in the reason).

To position correctly your parts, just use the snap tool, the rotation or position gizmos (arrows, planes and circles), the keyboard shortcuts (arrows mainly) for fast transformations. You can also do precise relative transformations on an axis. e.g. for a 42.5° rotation around the X axis of the selected point: make sure you are in local coordinates then press R, and X and type 42.5

Check this video too for snapping function:
[youtube]AsDczg1pOFc[/youtube]

To move multiple pieces, simply press Ctrl/Cmd and click the piece you want to had to your selection, then select the pivot point and rotate using one of the method above. You can even choose a pivot point outside of the selected pieces using the pivot point tool (P).

Thanks.
The problem I had was that I rotated a part, but when I want to snap it the workshop rotates it back to it´s original position, so I had to type in the number manually, and I REALLY don´t know which one´s correct. 😕

You shall post your particular case here (with pictures) so that I can give you your best option.

I´ve the problem that I´m used to a system where the parts fit without any little gap, perfectly. But here on Mecabricks (checked out your little Robin right now) there are gaps EVERYWHERE. So how should I know that the parts are fitting together?

And (I don´t know how to post pics) as an example: (Please) go to the workshop, looking for Bricklink parts and enter "52038" and place it. Now I want to snap a 3069b to the side studs, but I can´t snap it and I have to manually type in a number.
So if the 52038 is placed at "0 (by clicking on it MB displacs -3.918869928272537e-16) x 3.200000047683716 x -44" I don´t know if a "0 x 8.8 x -48" for the 3069b is correct.

Im really confused right now.

Edit: If you could have a look at my private projects, it´s 7737-1, there I already placed that.

The little gaps are so that the seams show up in renders for added realism I think. I know that gaps are intended. Scrubs said this: "Once the part is modelled, the next step is to alter the outside dimensions so that there will be a gap between the parts when they will be assembled:" See here: http://www.mecabricks.com/en/forum/topic/15

And for attaching images, take a look at this: http://www.mecabricks.com/en/forum/topic/14