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For the masochistic among us, here's the math for the 223 Rem...
First, we have to choose a representative standard deviation to evaluate:
Commercial 223 ammo I have tested has about 35 FPS standard deviation. My ordinary handloads have about a 25 FPS standard deviation. With a little work, I can get my handloads down in the teens, and with a lot of care, I can get them into single digits. So, for the example, pick 25 FPS as reasonably representative.
My Lee Perfect Powder Measure does pretty well at throwing ball powder charges. The standard deviation of the random error for that device is about .04 grains.
To a good approximation, a grain of powder in that cartridge is 100 FPS. So the standard deviation of the FPS error induced by switching from a perfect, ideal measuring system to the Lee Perfect Powder Measure is about .04x100 = 4 FPS.
Variation does not add linearly. To see the final result, you have to take the square root of the sum of the squares, so:
Final SD = (Ideal SD^2 + 4^2)^.5
Final SD= (25^2 + 4^2)^.5
Final SD = 25.3 FPS
Switching from an ideal, no error powder dispensing system to the Lee Perfect Powder measure increases the SD from 25 FPS to 25.3 FPS. That's nearly impossible to detect.
Doing the math again for an SD of 12, representing very carefully made ammunition, your SD increases from 12 to 12.65. Again, it's nearly impossible to detect.
A similar argument holds for pressure. The SAAMI specs have a safety margin built into them that assumes a 4% SD.
The day I did that math, I quit hand measuring powder. It's a CWOT.
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