I use a Ruger Magnum in .416 Rigby since 1994 and I can assure You that its trajectory is as flat as You will ever need. I sighted it in to be dead on at 100 meters and there is no need to even think about POI up to 250 meters, just aim and pull the trigger. I know the ballistic charts tell us otherwise, but sometimes reality is just different. I don`t know why but at least my rifle hits everything within 250 meters if I do my part.
BTW, the plan of Your PH concerning hunting no herd bulls sounds very good to me, hunting dugga boys is much more exciting than follow a herd and shoot a breeding youngster. And shooting a buffalo at long range is possible but whacking an old bull at close range is a thrill You will never forget !

Kalunga

Wicked good hunting !!!