My 2 bobs worth. With the ballistics you quote you have a 350mt rifle. Hold over from POI at 300 should be around 10 to 12" so about 18 for 350 if 12 for 300. Just rough in mind calculations. Only way you will know is to test it.

Now the important points, at least to me. If the 400's do not shoot to the same point of impact as your DG bullets then you need another scope. My preference would be for another rifle. I know my 416 Rigby is a 350mt rifle-not needed more than 15" hold over for the 400grain bullet and a 400 to 450mts rifle with the 340 grain Woodlieghs. However there point of impact is different. I would limit my self to the 350 mt. shots.

All this is dependent on whether you can shoot the rifle from various rests accurately.