Brightness of PWM controlled LED?
It doesn't work that way.

In one of the comments, you wrote:


  To achieve low power consumption, I want to make 64 LEDs look like they are all shining with the persistence of vision effect (high enough frequency). Because then, effectively only one LED is on at any point in time, which means the power consumption of 64 LEDs shining, is just the power of one. But I want the LEDs to have sufficient brightness, so they need to be at 4.5mA. Which means, 64 LEDs lighting up (perceived by humans) is actually only drawing 4.5mA. If all 64 was on at the same time, it would draw 4.5 * 64 = 288mA.


You want all 64 LEDs to light with the equivalent brightness of 4.5mA. You want to do this by pulsing every LED with 288mA at 1/64 duty cycle. So one LED at a time is on, 1/64th of the time, at 288mA, resulting in 4.5mA average current through each LED.

However, this means you have to supply 288mA at all times. 1/64th of the time for the first LED, then another 1/64th for the second LED, then the third, etc... You will be supplying 288mA continuously, you'll just be alternating which LED you send it to.

Meanwhile, if you just supply 4.5mA DC to every LED, you're consuming... Also 288mA, but in a simpler way. Possibly also more efficient, as LEDs tend to get less efficient at higher power.

I'm sorry, but there is no free lunch.

Getting more efficiency

There are some things you can do though:


Get better LEDs; efficiency varies wildly by model. Look for high-efficiency LEDs for lighting applications, not signal LEDs.
Put LEDs in series if your supply voltage allows it (judging by the supply in your schematic and blue LEDs, probably not).
Consider a current-regulated buck or boost converter instead of a current limiting resistor.
PWM, with the correct LED and at the right frequency can increase perceived brightness due to the way the human eye works. See this answer.


Other random comments

The 2N7000 is a poor choice for high-power PWM; it's an old signal MOSFET that is not well-suited for high currents. Its \$R_{DS(on)}\$ worse than 1Ω, maybe upwards of 10Ω in your situation. Use a power MOSFET.


  100mA to 300mA after testing, research and consulting the datasheet. This will give me an average current of 1.56mA to 4.69mA and the maximum I can get out of the LEDs without possibly harming them.


I'm not entirely sure how you mean this, but be sure to check if 288mA pulse current is OK for your LEDs. If the datasheet just says "4.5mA max", then 288mA pulsed at 1/64 is likely not OK.