Depending on the rifle and ammo you can need from 18-22 mils for 400 yards so I would buy a scope where you can get 22 mils out of it and then use a base that can allow the distribution of the allowable elevation in the scope to get that. With a scope like the SE which has 31 mils that would be about 15.5 up and down so you could use a 40 MOA base which is about 11.6 mils and redistribute so you has about 27 mils up and 4 mils down. That allows you to go to 400+ and not be maxed out at the top of travel.