How to use named formula

 Named formula will calculate automatically, see examples below

themeFill = If(_appTheme = "Dark",_appDarkFill, _appLightFill);

themeColor =If(_appTheme = "Dark", _appLightColor,_appDarkColor);

userScore = With({x:Sum(Filter(Posts, PostEmployeeID = _userID),Score)},If(x<0,0,x)) ;

Here instead of writing the condition again and again we written once for each control.

Now see below

selectedPost = First(Filter(Posts,PostID = _selectedItem.PostID));

avatar = LookUp(Avatars, ID = _userAvatarID1).Thumbnail.Small;

Whenver selectedPost is changed we gave the ;latest selectedPost object.