OK...have you ever taken a shot at a deer at 129 yards from a steep hill?
No. But I have at targets at 100, 125 and 150yds on a steep hill. And just throwing it in a ballistic calculator that can use angle doesn’t throw it off by much unless the shot is already almost missing.
https://shooterscalculator.com/ball...bf`Vel[x+y]~ft/s&lbl=&submitst=+Create+Graph+
Using the basic data for a .30-06, even shooting at a 29 degree angle, with a 100yd zero changes trajectory by less than an inch at 129yds vs shooting on the flat. Even with a 300yd zero it still only changes the POI by less than an inch.
Of course, if you’ve got a 300yd zero, the bullet is traveling over 5” above zero at around 120yds…which could be the problem if you are sighting your rifles for 300yd (or further) zeros and then not compensating close in. It is why I prefer using a BDC reticle or zeroing at 200yds and known hold overs, or for very long range (400+) a scope I could dial the dope on it using a laser range finder for the exact range. With a 200yd zero and most calibers you are fine to 300 with at most holding towards the top of their back when close to 300.
300yd zeros (or further) get really tricky at close ranges.
You mentioned 6” high at 100yds. You’d need to know the BC of the bullet and muzzle velocity, but that sounds pretty close to a 400yd zero. Just to emphasize, for accurate long range shooting, you need a BDC reticle that corresponds to the round being used and a laser rangefinder to know the exact range. Or a scope you can dial the dope on and a laser rangefinder. Otherwise it is just guessing and Kentucky Windage hoping you hit your target and kill it.
From 450 to 500yds on a 150gr .30-06 hunting load may be greater than 12” of drop.