I'm sure you already know this, but for those who aren't sure/aware of DS rules:
- Only the first model in the unit is placed when you choose the location to DS.
- Next you would measure distance between this model and the icon if there is one.
- If not, or if you're too far away from it, you then roll to scatter the single model.
- Once it's position is final, only then do you ever fill in the rest of the unit.
In cases where you are near other models/terrain/table edge, you can choose which half of the outer circle to populate, which can help avoid mishaps, but only really helps if a) there is a partial outer ring and b) 1" would make a difference. This is a small point but can make the difference on a crowded table. Remember also that terrain does not cause a mishap unless it is impassable. Remember also that you only need to be 1" away from enemy models - anything else can be within 1" so long as you don't actually overlap.
In more specific answer to your questions:
The general rule in 5th Ed when measuring distances (except for movement) is to measure the gap in between the 2 objects/units/models involved. In this case i would say the DS model does not have to be fully within 6", but instead must just be partially within i.e. gap <= 6" .
Interestingly, the Movement rules in the BRB state that you are allowed to measure movement distances as soon as you have declared that the unit will move, but you don't have to commit to any particular direction or range. Whilst this does not relate at all to DS, i believe it is intended to give a certain amount of tactical freedom in where units are ultimately placed, but perhaps it is simply because it is too difficult/nitpicky to regulate if the unit ended up exactly where first stated
If it is indeed intended to be deliberately lenient when dealing with movement, i think the same leniency ought to be extended to DS (in relation to icons, beacons, etc). Specifically, you should be allowed to say "I'm going to DS to that icon" and be allowed to place the model anywhere within 6". Given their nature, it seems pretty difficult to miss a great psychic beacon shining out to you in the darkness...
"Adept, lock on to the Captain's homer and deploy 1st squad in a protective position at his side."
"Done. Energising now."
"Whoosh!... Waaaarrrghhh!!! .... .... ...!"
"Oh, wait, did you say homer? I thought you said large rock. Oops, my bad."
It's really not going to happen
(Yes i know it should be daemons but i just can't picture them standing around in the Warp discussing what they should do upon arrival. If anything it's even less likely that daemons would miss an icon as they should be drawn inexorably towards it whether they want to or not! Plus it's funnier when terminators get lost in the Warp
)