I'm a firm believer in Varmint Hunting as a training venue for long range shooting. When some one ask me what is the best training you can do for the sniper craft, I say Varmint Hunting.
This is right out of the USAMU's Sniper Guide:
Chapter 5 General Notes:
1. ….. The counter-sniper is a hunter and must use any and all tricks of the trade to assure a proper hit. The lives of fellow officers and that of the general public are at stake. Time is extremely critical, therefore, he can expect to be required to make shots at varying angles and distances on a split second’s notice. The hunting of varmints such as woodchucks, prairie dogs, and crows provides outstanding training because the techniques involved are almost identical.
Ok lets assume that's correct (and I believe it is) we'll move to the question;
Wouldn't you be using a much higher magnification device to find dogs at those ranges? You would need to find a much larger object to range and use that as your reference. Don't see how you could hold steady enough to range something that small at 600 yards
PD's live in PD towns, the towns are littered with little mounds of dirt around their holes, most dogs are killed on or near these mounds. The mounds are much larger then the dog itself.
As you lead to in your question, it seems to be easier to range a larger target then a small one. You don't really need to go to a higher magnification to range the dog if you can range the much larger mound.
You almost always can find something to range near enough your target. Sure you may be a few inches off, but you're gonna be pretty dern close. You can also use a map to range your PDs, you have hills, dips, ridges, etc etc that can be found on most good maps. If you know your location, its pretty easy to range the PD holes using nothing more then a map and some sort of protractor (such as comes on most compus).
A laser range finder is quicker, a Terrrapin, will lock on your PD mound, where at 600 yards most of the $500 models wont (or at least the ones I've tried).