i do this for myself and ask myself ( why not share with others ?)
i have enougth to place object in the CK i need use a calculator for place it exactly good in world and not (flying on the table or mix with the floor)
this way is the best way for place object with ultra minimum of chance to have this problem.
i have tested it with mods, vanilla game and lots of stuff.
all the details is not very necessary and some things can be not necessary do, but i higly recommend to follow this steps.
sadly this need to be again everytime you start the CK (not my fault) but its pretty quick and simple to do and take like10-20 secondes to do.

at very first you need start the CK (load it completly of course) and load your mod (or skyrim.esm or whatever).
in this exemple i use the mod "skyrim monster mod" by dogtown1, and use the Bullvore for ragdoll.

when the CK is load and your mod too, select a place in the cell view (in my exemple i use the dragonreach castle), double clic on it in the cell view for show it in the preview window.

now, make a right clic on the preview window and select "Render Window Properties", clic it.

- select the "Render Window" tab, check the "show memory usage frame" and clic "apply"
- always in this window go in the "LOD" tab and put all the left sliders to max ( put it in 2.00) and check the "enable landscape LOD", and click "apply" again (its normal if all dat sliders return into initial position after, dont worry)

- in same time go in the "preview movement" tab and change the Zoom Speed from 0.05 to 0.08 (because its more easy with this setting ^^) clic "apply" and close this window
- now go in the "object window" and select whatever you want to ragdoll (((( i higly recommend to use a npc or creature at first try))). in my exemple i select the skymomod bullvore. clic and drag it into the preview window. the npc/object/creature should appear in the window where you have exactly put it.
- now, let see if it work ^^, if you have selected an npc or creature (highly recommended for test) just double clic on it in the preview window, thats open the npc/creature "properties" window. at down of this window, check on "start dead",
and.......when you clic on "OK" you should see immediatly the npc/creature fall like a ragdoll , like when it die in the game ...
congrats ^^ thats work, now you can do this with all object (except static object and too big object) all the time you stay in your party.

- you can do the test with "cabbage" (its nice exemple) , the first one you place should move itself for take a good position and never "mix" with the floor or any wall/object around.
its very nice for place object correctly in the mods or simply obtain an "realistic" situation about something supposed to fall from the air. if you place another cabbage on the first one, the second one should fall and roll away

CAREFULL !! if you place an object near a mountain, you have chance the object go away like crazy and you need search for it in the down of the mountain ^^

!!! make a skeleton turn into ragdoll make it fall in 1 piece and not explode in all bones !!!

!!! if you place a object in air, it automatically fall on floor , be sure to check "don't havok settle" for make it stay in air !!!

!!! dragon can be turn in ragdoll, they fall die but not turn into bones !!!

!!! lots of creature/npc can have blood on them , thats normal, dont worry !!!

i hope this help some others modders and sorry if my english is absolutly poor and full of error , i do my best XDD