I don’t think you’ll find a chronograph at the airport to check your loads.
Just make a “factory label” sticker that lists whatever numbers are needed to make the Namibia government happy and paste it on your hand load’s ammo box. Have fun with the name, “Ol’ Off-to-Hunt Ammo Company.”
Make the loads, keep them safe in your rifle, and go enjoy your hunt. The buffalo isn’t going to know if you have 5400 joules or 5300. Having the same point of impact with the TSX and Hydros would concern me more.
