When to choose a Normal attack instead of a Full attack
In Starfinder you get to choose between a Normal attack and a "Full" attack. Basically attacking once or attacking twice with a -4 penalty on each attack.
I've gone ahead and done the math for you and can tell you that if you need to roll 13 - 17 on a d20 to hit then you should opt for a Normal attack ... otherwise go full out, unless you're short on ammo.
Here's a chart with a bit of visual proof. I won't bother you with the actual math :)
A simple example
Let's take an example where you want to hit something AC 16 with a tactical semi-auto pistol. And let's assume you've got no bonus to hit, so you really do need a 16 or above to hit. Use this Dice Calculator and punch in:
(d20 AC 16) * (1d6) crit (2d6)
That's an average of 1.05 damage per attack; pretty miserable.
Now let's do the math if you tried a full attack. With a +4 penalty you need a natural 20. You still get a chance at critical damage because 20 meets or exceeds the AC you're trying to beat. Let's punch it in:
(d20 AC 20) * (1d6) crit (2d6)
That's an average of 0.35 damage per attack, and you've wasted twice as much ammo ... terrible!