Down Lights location- help

Hi evryone, i need some help...
I am trying to whrite some excel file, to calculate Down Lights location above ceiling.
I need to put distances from wall to first joint, from wall to second joint, from wall to therd joint,... etc. to last one.
I need to put information, how many lights i want to put in one row.
I need to put information about dimension of joints.
I need to put information about dimension of lights.
I want from excel to calculate distance from wall to first light and distance fom last lamp to wall.
I want from excel to calculate distance betwean lights.
Excel must calculate location and consider location of joints, to avoid overlap.
Sorry for my english)
Thanx for any help guys.

A sample file showing what you're trying to accomplish would be helpful.

I am begginer in excel that`s why i am asking for help.