The spreadsheet will simply tell you to gem the best thing possible for 'this' item. It won't tell you where is the best place in all your gear to put your nightmare tear.
For your gear, just put the nightmare tear in the chestpiece. As for gemming primarily for haste, that's a little more tricky. Go back to the sheet, and actually plug in all haste gems in yellows, and AP/haste in orange. You'll see that the sheet likely recommends for you to gem back to AP. The more of a particular stat you have, the better the others get (with the exception of ArP, but that's another matter). I just got to the point with my gear where I can gem for mostly haste, and still have the sheet recommend more haste. My rogue
Aside from being over expertise cap because of your boots, things are looking pretty good. The difference between 4pc T10.5 and 2pc (gloves/pants) and LDW shoulders/princes head is pretty small, and I find the playstyle of not needing to worry about the 4pc proc much easier to deal with. If you get the chance to get both pieces (3pc + offpiece doesn't make up for it), you might find it easier to work with.