Sorry, but i do a lot of long range shooting - tactical competitor and f/tr etc. I also do as much stalking as i can fit in ( i can expect to put all 20 shots in a four inch group at this range but i cannot garantee that my 1st sighter will be within 4" of the centre of the v bull though) neither can world champion shots!
And honestly 300 win mag will not make you shoot any better, indeed it will probebly make you shoot worse due to the re-coil. .260 remmington, 6.5 x 284, or one of the 7mm calibres should be a better choice for flat shooting and bucking the wind than 30 cal anyway.
Please, please, please do not shoot at these distances - putting in 4" groups at 400 yds without sighters is way, way harder than 1" at 100!
besides which deer can move a heck of a great deal when the bullet is on its way to target even if you shot was released perfectly with the ideal dope!
Anyway if i had to make the shot i should range it with a sepparate range finder and use a Hendzolt ? (military Zies) Nightforce or Leupold etc scope