The best rule is what shoots best but 85 and 90 sound like alot in a .50 I only shoot 70 ffg in my .58 and I've never had a ball stop in a deer. I guess it depends on the pitch of twist.
Its not a power thing or a "mine is bigger than yours" thing, its accuracy and trajectory. Low velocity balls are as likely the stay in as high velocity. A 490 RB at less than 800 fps will shoot though a Mule Deer or antelope chest I have done it twice with a pistol that only made 800 at the muzzle. This penetration is comparable to a 50 caliber rifle with a 90 gr load at about 200 yards based on tests I did years ago. 200 is too far to shoot animals with a 50 caliber RB unless in a situation where its worth the risk. But 150 is very doable if the load is fast enough and the rifle is sighted for 120-130 yards. But in the woods in the east its not needed.
A friend of mine has actually hunted with an original Hawken rifle that was used in the west and I had mentioned that this rifle has a fairly low front sight and a typical high late Hawken rear and a tapered barrel.
He said "funny you should mention that" and related shooting the rifle years ago and killing a deer with it. He thought is was probably zeroed for 160 yards.
Hunters in the east really have little idea how tough it can be to get with 100 yards of Antelope or even Mule deer some times. I have shot deer at 300 with a modern simply because it was impossible to get closer. I crawled about 50 yards on my belly to get that close.
So I tend to use flat shooting loads in my ML arms. I kill a lot of animals under 100 but there are times when its simply not possible and having an extra 30-50 yards of point blank range is important.
But a high velocity load, like 90 gr of FFF in a 50, will let me hold center of the deers chest from 0 to about 135 yards. Hold a little high and its good for 150.
But the shooter has to be up to the shot.