Walls and Ceilings - Drop Ceiling - offset from installed Light fixtures

Exactly where should the drop-ceiling main spine and perimeter molding be installed relative to the "hi-hat" ceiling fixtures lowest edge ?
1) Exactly at the "lip" of the light fixture ?
2) 1/2" below the "lip" ?
3) lower than that ?

Your ceiling tile needs to be able to slip under the light fixture. 1/2" below the lip should be fine, but you might have to push up on the fixture when you go to install the tile that goes below it. If the light fixture is fixed, you need to stay 3/4" - 1" below the lip, since it would be impossibile to slip the tile in otherwise. Sometimes it's easier to loosen the fixture altogether... put the tile in, then reinstall the fixture. Many of those lights can be raised and lowered by a simple adjustment. The light fixture should not be putting any pressure on the tile.

and this may be my fault....I was trying to be clever and I used 1" aluminum strapping screwed to the above floor joists as my support mechanism....I didn't want the lights to be moving and clanging/banging around. I achieved that objective with the trade-off of making the tile installation a pain !! Should I have just used 12 guage hanging wire instead ?

Thanks much......one more question....the room is L-shaped (bathroom, natch) with one leg 55" in width. I heard it's important NOT to have a small cut of ceiling tile....that I should move the main beam such that the one small "leftover" is now two larger "leftovers". Is this true ? My small leftover was going to be in the outer edge.

Most lights like that have their own strapping that slide past each other when fully contracted or fully expanded. There is a slot near the lower side of the can where this strapping slides through. I don't think I've ever seen one without it.

Regarding the grid layout, if you send me your bathroom dimensions, and wall lengths, I can send you a plan.

http://www.javazon.com/test/BathroomRenovation-ceiling.gif

You'll note how my layout of the grid keeps a 2 foot cross member right along the inside of the "L"....but now there are small left-overs at the outer edge.

Also note how I have a "cut-corner" due to a drain-pipe.

Thanks much for your advice !

Hmmm....looks like I should just "shift" the entire layout to the right about a foot to remove that rightside leftover.....but then the grid support is not aligned with the "L"....

Here's what my program comes up with, although I tweaked it 1" north.

http://pg.photos.yahoo.com/ph/thexsleeper/detail?.dir=af9d&.dnm=32e6.jpg&.src=ph

again, thanks much....I am using that only with the right-side smaller due to constraints of fan and light placement.
Sheesh....I learned quickly how inter-twined the light and fan placement are with the ceiling tile !!!

A layout diagram is in this post:
http://forum.doityourself.com/showthread.php?p=821580#post821580

Boy, this final step of installing this drop-ceiling is REALLY A PAIN...there is no margin for any error.