Here is a system for quickly knowing what to go for once you are at or below 100, when playing 501.
Subtract 40 from what you have left. The result is that number of points you need to score in order to leave yourself with a double 20.
If the result after subtracting 40 is not a triple, then add 4 to the result. This would be what you would need to leave yourself with a 36 for a double 18.
And if that is not a triple, add 4 again which is what you would need for a 32 or double 16.
Here are some examples:
Score = 97 and 97 - 40 = 57
57 = Triple 19, so the 2-dart out is triple 19, double 20.
You throw for the triple 19 and miss with a single 7 so your score is now 90.
Score = 90 and 90 - 40 = 50
50 is not a triple, so 50 + 4 = 54, so the 2-dart out is triple 18, double 18.
You throw for the triple 18 and miss with a single 4, so your score is now 86
Score = 86 and 86 - 40 = 46
46 is not a triple, so 46 + 4 = 50, which is not a triple, so 50 + 4 = 54, so the 2-dart out is triple 18, double 16.