I'm with Andy on this.
It depends on a couple of things:
1. The side wall of your tire (rounded or vertical, this is also partially determined by your tire sizing)
2. Suspension setup - If you are lowered the tires tend to camber in a bit, especially in the rear. The lower you go the more pronounced this effect becomes. If you buy a camber kit you can bring this back into spec. this lead me to the next item
3. Camber - how much camber you leave determines how far you need to push the tires out to get flush (I have about 2 degrees in the back, running H&R's with no camber kit)
4. What you define as flush - people have different determinations of what flush it. I like the top of my tire to line up with the fender well. Others like the middle of the tire to line up with the middle of the fenderwell, etc.
5. If you are running the stock mudflaps - This makes a pronounce difference in how flush your look is, without the mud flaps you can get away with a smaller spacer IMO.
With all that said, usually on the first gen FX I see people saying that they are going to run a bigger spacer out back. Which is almost 100% wrong. The front wheel on the FX sit much farther in board than the rear (~5mm). I could only test on the stock 18" setup, but I believe the optimal setup for the stock 18's at stock height to be: 25 in the rear, 30 up front. This is given by the fact that H&R doesnt make half sizes like Ichiba. They dropped them quite awhile back, and it makes sense, think about how little difference there is between 25 and 27mm. It is completely irrelevant...
With that said if you have the option of the Ichiba's and are hell bent on getting 27mm sizing the optimal would still be a similar setup: 25 or 27 out back, and 27 or 30mm in the front.
I believe these setups to be very similar for once you have the H&R lowering springs; however, once you move to coilovers and you start to go lower, another size up might be needed.
THIS IS MY OPINION FOR THE STOCK 18's, I DO NOT KNOW ABOUT THE 20's. Although they are the same offsets the tire bulge is very different on the 20's and I suspect a larger size might not be completely out of the question.
