If you look in the chart above, we sampled one man's platnum ring of medium to large size and it read 157. If you look in the "Condensed" thread link near the end, another similar sized platnum ring read very near it in numbers if I remember right. I would shoot for perhaps nickel to tab zone (like 140 to 169) perhaps for a large women's platnum ring maybe then.
Of course rings can read wildly. Even gold rings of the same K and size can read off a good bit, due to what they mix with the gold (copper, nickel, silver, etc). I don't know about platnum. Is it mixed with other metals or is it pure? If it's pure, then the numbers should be very close based on size from one to the next.
Either way, might be fun to bet on VDI #s as a game here? What do you mean by "thick"? Meaning as wide as a pencil's width? Not thickness, but width. If that's the case (that wide or wider), then I'll guess minimum of a 140 VDI # but more than likely in the low 150's. I'll bet 151 as the VDI #. Anybody else have a guess?

You WILL find that ring. Just a question of when, since nobody has hunted that yard, unless the guy came back and found it in the snow, which I doubt. Good luck and keep us informed!