I’ve hunted nyala, bushbuck and buffalo with iron sights. The nyala and bushbuck were with a 318 DR and the buffalo with a 425 bolt gun. I don’t really know the distance on any, but they were all under 100 yards and the nyala was quite close. I also used the 318 DR on a bushpig over hounds hunt, which was quite an adventure.
Rule #1 above is spot on. You have to be ready and comfortable to pass on game that you could have taken if you were hunting with a scope. Your PH does too - there will likely be less in the salt and fewer trophy fees when its all done - doesn’t mean its a less rewarding hunt.
You and your PH need to know it’s not just about range or distance. You won’t take the shots into small gaps in heavy cover, or looking into the sun, that you would with a scope. You really need the sun at your back or overhead, and better target presentation, or simple get really close.