ABS operation with different size tires

I have a '95 Impala SS and am wanting to put smaller width/diameter tires up front. My rims are 18x8 front and 18x9.5 rear. Rear tires are Goodyear Eagle F1 295/45/18. I'd like to run a 265 or 275/40/18 up front, but I want the ABS to continue working. Is there a definitive percentage or ratio at which the ABS will become inoperative with different tire diameters?  I haven't found a very scientific answer to this yet. I'd really like to know before ordering my tires!  

I haven't seen any hard evidence of tire size being a serios factor in the abs system. My buddy is running 315/35/17 on the rear and 275/40/17 on the front of his 97 z28; and his had no affect on the funcionality of the abs system. And we have put it through some ruff tests and had no problems. we have gone as small as 245/50/16 on the front and had no real affect on the system. the small amount your trying to change isn't going to hinder your performance of the system. as far as a difinitive percentage i haven't seen a chart on it or anything yet. but if you find one let me know.  

Only difference in height will effect the ABS. Like running slicks and skinnies on an F-body. A drag tire will be taller than the 165r15 radial in the front. The ABS system becomes confused and shuts down.

I haven't heard of people running 17s in the front and 18s in the rear having this problem, and I feel that is because at 275/35-18 is very close in height to a 275/40-17...