Re: Ring Height?
Essentially in your case yes.
With your current set up, if your zeroed at 100 you will run out of adjustment at about 600m if your shooting 308 and a 20 moa rail will allow you to shoot to beyond 1000m.
It just gives you much more adjustment up.
The biggest problem with most setups is the scope adjustment is in the middle of of the scope at about 100m and you will never ever need the bottom half of your scopes adjustment range, so adding a 20moa scope lets you use some of this wasted adjustment.