Forced carb question- over carbing

I have been reading a lot of posts about forced carbing. I am in the process of getting a system using commercial quarter kegs.

Now I think a quarter holds 6.5-7 gallons and just starting I only have 5 gallons to fill it up with.

What is the process or what are the precautions of over carbing with that amount of head space?

Just carb to the volume at temp of the style of beer your kegging head space has nothing to do with over carbing.