RMc, that's a sound question that I don't have personal empirical evidence to answer. My frame of reference is purely anecdotal, coming from the field. South Mississippi deer habitat pine woods with a lot of brush and undergrowth, or in hardwood creek bottoms. In both situations, range is very short. In the hunting club I was in that did run dogs, I shot quite a few deer with 00B and saw a lot more being cleaned in the cleaning shed. Most of the deer had at least 4 pellets or 5 pellets in the vitals, usually more. Those with less were usually bayed by the dogs after long runs, sometimes miles (that club had 4000 acres under lease and Walker hounds can cover a lot of territory very quickly).
I do not think that the frequently cited "1,000 foot pounds of energy" formula for rifle bullets translates well to multiple projectile shells like buckshot, because 4 or 5 pellets are still going to be well below that threshold, and a single pellet to the brain is fatal. #1B will have less energy, but typically patterns better, at least for me in my gun (more about that later).
It also probably matters a little whether you're shooting at a standing still deer, or if you are shooting at one running full speed ahead of a pack of dogs and you're trying to estimate lead and shot string on a moving target, like wingshooting. But, clearly the more modern loads with Flight Control wads will hold a tighter pattern and have a better chance of getting at least 5 or more pellets in the vitals. Our deer here have smaller bodies, although antlers can still be impressive. A typical whitetail buck will not exceed 200 lbs, so the vitals are correspondingly smaller.
Another factor to consider is that different barrels pattern buckshot differently. You MUST pattern buckshot in YOUR barrel to identify which load and buckshot size works best. Frankly, I've not patterned recently beyond 15 yards, because I'm not hunting with buckshot these days. I reserve it for HD, where range will never exceed 15 yards and will more likely be only half of that.
So, my empirical evidence for pattern density and range is borrowed:http://www.theboxotruth.com/docs/bot56_2.htm
If that data is accurate and typical of most 12 gauge barrels, then 35 yards is scary poor for "ordinary" buckshot and still pretty good for #1B Federal Flight Control. To answer your question, based on that data, "conventional" buckshot would not meet my 4-5 pellet criteria beyond about 20 yards, and that is consistent with my anecdotal experience. I'd take a 35 yard shot with Flight Control #1B, but you can see that the pattern is starting to open up. 50 yards MAY be the safe limit to put 4-5 00B Federal FCW load in a small whitetail vitals reliably, but that is speculation on my part as to range.