Help to lay out lights

Hi all, I am replacing my 1000w lights with 600w lights. Ceiling is 7 3/4'. I am looking to light the entire floor area as opposed to just the rows of plants, for better uniformity. Im using gavitas 600w se. The section of room is 13'x18'. I will run two rows of (3) 2'x8''drain tables across the width of the room. I've read these lights are good for an area of 3'6" between the fixtures and 6' between each row. I think I'll need 8 or 10. My problem is knowing how to space them properly. It's not for lack of research. This kinda thing just doesn't come that easy for me. I am hoping someone can help explain this in a way that I can understand? If so..... THANKS!