At 200 yards I increase the target size so that the crosshair blocks all the central white so I know I'm centered. At 300 I do the same thing but use the first ballistic dot to obliterate the white central enlarged dot.
It sounds to me like you are quite experienced and know what you are doing.
But reading the above quote seems to reveal you adjust the reticle size by adjusting the magnification.
The center crosshair is not offset,so varying the power will not affect zero,
but the dots ARE offset from the center crosshair .When you adjust the coverage of a target feature, you also adjust the offset of the dot. You are .in effect,adjusting the elevation by an unknown amount.
If you zero that dot dead on at 300 yds using 10X the bullet will strike a different elevation at 8x or 14 X.
The 25-06 is just fine at 300 yds on a deer.
Consider the regard given the .257 Weatherby and .264 Win Mag. Yes,there is SOME difference. But not all that much.
Looking from a different direction, I have NO doubt my .257 AI ,or a 6.5 Manbun,or a 243, or a .260 Rem will cleanly kill a deer at 300 yds. All of them are adequate. Don't let your gun club buddies sell you on a 300 Win Mag.
One way or another, I doubt your bullet hit the heart/lung area. The heart and lungs would have poured out as chunky soup.And the deer would have dropped.
I have no doubt you can shoot.
It seems to me your method of compensating for your vision problem is faulty. And quite correctable! Your drop compensation is only valid at one magnification,whatever that may be.