I am looking to specify ball mill discharge size for ground lime to be added to process water.  Does an 85% minus 200 mesh make sense?  Please comment.

85%-200 mesh is a fair size, however there is a lot of speculation whether limstone does any good, as it does not absorb acid. It merely absorbs (hence need large surface area) SO2 and emits CO2

see  http://www.digital-analysis.com/Limestone.htm\

The fineness of hydrated lime is not due to grinding:
Hydrated lime appears to have a fineness (particle size) not unlike that of flour. Limestone (calcium carbonate), which is used to produce quicklime (calcium oxide), has a rhombohedral crystal structure. The conversion of calcium carbonate to calcium oxide (quicklime) results in a cubic crystal structure. (Carbon dioxide is driven out of the carbonate portion of the crystal structure by intense heat energy, and a Ca-O bond remains.) When quicklime reacts with water, the resultant hydrated lime (calcium hydroxide) has a hexagonal crystal structure which occupies about 25% more volume than the original cubic structure of quicklime. As the quicklime-water reaction takes place, you can see actually see the quicklime pebbles swell and occupy more volume (if the ratio of water to quicklime is low). Mechanical means are used to remove the normal grit that results from lime being produced from a naturally occurring material, however, the particle size of the hydrated lime is generally the result of the formation of lime from the reaction of calcium oxide and water and the resultant shift in crystal structure from the formation of calcium hydroxide.

http://www.cheneylime.com/limefact.htm

Thanks Bimr,  I would agree with you 100% that lime is an ideal product (and air seperated lime is very flour like).
Jackofallmech had asked about using unprocessed "LIMESTONE" this is a much different material and not one I would recomment for pH control.