Hi John,
There are a few ways to do this.
Using a texture:
This may require a bit of experimenting but you have to adjust the texture you are using, so that the 'rails' in the texture are sized to suit your grid layout, using the Mapping control on the Appearance Tab. It is then a question of moving the light fitting into position manually by pointing a camera at the ceiling. A little tedious but 'do-able'.
Using supplied texture:
Essentially using the textures below it is the same process, but I have created a Centred texture - you will find this is needed to centre the grid in the ceiling easily, which you will need so that you lights are also centred. Set the texture scale in the Material list to the length of your light and it should fit. Still the same issue trying to align the lights, but if you insert them using the right click Array they should be exact placing and so the texture, if sized correctly should match (or you may need to adjust the scale slightly).
Using actual grid of members:
The third option is to insert an actual grid using members (see image). This is not as hard as it seems.
1 ) Use the Layout toolbar 'Line' tool to draw a line representing one rail in your grid.
2 ) Right click it and choose 'Convert To Members'.
3 ) Choose a member shape (eg T upside down) or just the flat bottom rail if you want, since that is all that is seen.
4.) Right click and Elevate to the ceiling height minus a fraction (so it is just below the ceiling).
5 ) Right click again, choose Array and enter Rows 1, Columns (number of rails) and the distance apart (eg 4'). (OK will enter all the rails for you exactly placed)
6 ) Select one, Right click Duplicate and drag it aside.
7 ) Select the blue triangle and rotate it 90 degrees.
8 ) Shorten or lengthen as needed (drag end)
9 ) Right click and set in Array again, Columns 1, Rows (the number required) and the distance apart (eg 2') and you have the opposite grid.
10) If you have a wall shape other than a rectangle, adjust the lengths of the affected rails as necessary.
11) Position a light in the lower left corner of the grid exactly (you can do it from overhead if you turn off the ceiling and roof)
12) Select the light fitting and again use the array to create the number of rows and columns of lights and the correct distance apart. (They should all end up in the right place in the grid - hopefully!)
If needed, dupicate one of the members, shorten it to the length of the light and position it on the other side of the light (if it takes up half the grid spacing as in my image - although I have not done this in this image). Again you can Array it, or with Ortho (status bar) on, duplicate it and drag to next light. Repeat until one row is completed. Select all these short pience and Duplicate and drag to the next row and so on to each row has the short member.
Obviously the last method increases the polygon count but not too dramatically.
Hope that helps.
Allan
