sump question

[mandarin_watcher]

Hello, I am in the process of setting up a 180 gal tank, I am going to use an old 75 gal for my sump. I have ordered an ASM G4 plus for my skimmer.
This is my first sump. and my first time DIY so I just wanted to get a few opinions on my plan. The sump is going to be a sump/fuge. I would like to get as much fuge as I can. So if I can go smaller on some area's without causeing problems in the future that will be a plus.
The inside diam. of the 75 gal is 17 1/2 x 47 1/2 . My plan for the baffles are as follows:
16 2 4 6 10 2 7 1/2
skimmer baffles carbon Live fuge baffles returm
ASM G4 + filtration Rock area MAG 24

The skimmer area I planned on have it 16 inches deep, then go down to 15, 14 and finally 13 in the return area. I'm not sure if I can cut the skimmer area down. The footprint of the skimmer will be 11.5X16. I will have the skimmer sitting up on somthing so the water level on the skimmer will be 8 to 10 inches.
But I also want enough open space in the sump area incase power outage or other problem. I don't want to chance the back flow FLOWING out on the floor.