I recently got back from my 1st NZ trip (stunning country). I was just taking normal holiday snaps but wished to get background as well as subject (person) in focus. We were on top of a hill and background was the beautiful harbour and then city then mountains in background. Just a beautiful vista and it would have been great to get at least the harbour in better focus. The photo covered subject but just nothing much else. So ultimately it was a fail. I used f9 on a 55mm focal length and was about 7 metres from subject.

Since getting back and investing how I may have done better it seems I may have been better with shorter FL and higher f number maybe to around f18.

When I looked at a DOF calculator it seems that there is a point at which the DOF combination causes a jump for near distance to infinity and you cannot get the in between distances - if that makes sense. For instance, in my shot it would have been good to get good focus to other side of harbour and maybe into city and accept rest as background. This would mean that I needed about 6 metres to maybe 3 klm.

Does this then translate to infinity to get the harbour in more focus?
In the DOF calculator I set my camera with say f18 and subject distance to 6 meters. When I set FL to 45 it gives DOF out to 150m while reducing FL to 45 means it goes to infinity. Is this the case?
Am I understanding this correctly?