A lot of players learn the very efficient trick of adding their score by counting up each hole as the amount over/under three for each hole and then taking that offset vs. 54.
- E.g. if your card reads: (3 3 3 3 2 3 3 3 3 2 3 3 3 4 3 3 5 3) you'd go across in your head, ignoring each three, and adding as such: -1 (first "2"), -2 (second "2"), -1 (first "4"), +1 (first "5"), 54+1 = 55.
- The gotcha is that players do this so often that when they play a layout that isn't 18 holes they forget that the base is no longer 54. If you play 21 holes you take the offset vs. 63. It's a not uncommon mistake when playing non-18-hole layouts.
Good stuff, it's a handy trick to know and definitely a time saver and away that's less likely to come up with the wrong number (less arithmetic involved). One caution though, is that I've seen people use that and then write down something like -2 for the score. Make sure you write down the actual number of strokes it took you to complete the round.