Exactly. Subtract the spacer width from the wheel ET and you will end up with your new ET.
To get that wide look your looking for, there is a few things you can do. Reducing the ET is one of them. This will stick your wheels out further, preferably flush, or almost flush, with the arch. Couple this with wide wheels, preferably dished, and run a slight stretch (e.g. 225/40/18 tyres on a 8 - 8.5" wide wheel), will all help to give the impression of width.
Of course, there is a lot to be said for running the correct tyre size for the wheel also. I would guess your wheels are 7.5" wide, so the above mentioned size is perfect. Obviously you dont need a dished wheel, but I think it always helps the illusion. Look at these:
They certainly help create the illusion of width, coupled with how low the car sits.
On the other hand, that all costs money, and looks best when your car is very low. But then you will loose some drivability / comfort in doing so. So its up to you how far you want to take it. I like the look of your motor already, and would probably only consider a pair of spacer on the rear. Maybe 8-10mm. I think I would get 8mm first. Because if you then think you need more, then you can put the 8's on the front, and get 10's for the rear. Or something similar.
Up to you though, and what I have said is only my opinion. There are other opinions out there, so try doing a little research and see what conclusions you come up with.
