Doc, Im sure this isnt the answer you want to hear, but the only sure fired way to know, is to shoot it. Having (2) 7mm myself I can guess and say you are probably ABOUT -5" at 300. Barrell length, FPS, wind, all come into play when shooting, and for someone to give an exact answer isnt going to be exactly right.

*My 7mm mag Ruger M77 Stainless 24" bar. zeroed at +1.5 @100 shooting handloads at 3080fps give me -8@ 300yrds tested