This is my approach on the 416 Rigby. I sight in at 50 and check that at 100. It's close. If I had a follow-up on a wounded animal beyond that, I would hold a bit higher and send it. I had that happen with a buff in July. Follow up shots were probably 120-140 and I just held a bit higher and had no problems.50 yards. This is a dangerous game gun, so you should be that close or closer. And if you're dead on at 50, you will be near enough at 100 that you won't have to adjust at all.
If you're taking long shots with your .416, I'd think it was time to get another rifle!