There are three rules changes that need to be addressed when scoring a game using Home Field ScorebooK:
Designated hitter in the National League
For consistency and memory savings, the app uses a common rules set for all games in a league that cannot be edited once selected. For this reason, simply editing the League Rules to enable the DH will have a slight ripple effect on games from past seasons (mostly box score and scorecard layout.)
To avoid this, create a new league called "National League 2020" with DH on and add your existing NL teams to it. (In a future release with Seasons turned on, you will be able to merge them back together.)
Automatic runner on second base in extra innings
This feature has long been requested by softball and baseball tournament fans. When the first batter comes to the plate in each half of extra innings, tapping 'Sub' in the upper right will bring up a new 'Place Runner On Base' action to select which player should go to which base (or bases.)
The rules are different for each league, but in the MLB in 2020 the player in the batting order before the leadoff hitter in the inning goes to second base -- with one exception. If that batter is the pitcher (rare because of universal DH) then the batter before him can be selected as the automatic runner.
This one is trickier still -- when setting up the games of a short doubleheader, do not use the league rules when prompted. Instead, set up custom game rules with 7 innings (and don't forget the DH!) This is necessary to enable the runner-on-second tie-breaker to come on the "extra" 8th or 9th innings.