A Formula.

Let’s assume you have a strong feeling about who should be president (or who shouldn’t). Let’s also assume you’re ticked off at the whole political process.
Let’s assume that the current situation doesn’t change wildly between now and election day.

We have Hillary Clinton in the lead, but still capable of losing.
We have Donald Trump behind, but still a possible winner.
We have Gary Johnson holding between 8 and 15% as the Libertarian Party candidate.
We have Jill stein at 0.5 to 5% as the Green Party candidate.

The reality is that either Trump or Clinton will win. Stein and Johnson are running to make a point.

With this scenario, you have options. Let’s assume that you want to make a political statement but you don’t want the wrong person to get in the White house.

So, if you are living in a bright red or blue state, you can vote Green or Libertarian because a small or even a medium number of votes won’t change the outcome for that state. (Example, you live in Maryland or Texas).
If, however you live in a state where a sudden swing of a small number of votes would switch it from blue to red or the reverse, you have to be serious and vote for or against one of the top two candidates.
Florida, in the final count in 2000, went for Bush (coincidentally enough the brother of the Florida Governor) by 538 votes — thus deciding the presidency and putting us on a track to invade Iraq.

So, see where your state is and decide how you can afford to vote.