Here we go again....
Black powder propellants (whether real black powder or synthetic/substitute black powder) are measured by VOLUME. Period. End of story.
Real black powder measured by volume will be almost the same amount of powder as real black powder measured by weight; eg, 80 grains by volume of black powder is approximately 80 grains by weight of black powder. This is NOT true for the synthetic/substitute black powders.
Having said all that, it is also true that many commercially available volumetric powder measures are NOT PROPERLY CALIBRATED and therefore do not accurately measure powder by volume. I have several different volumetric measures, and the two most extreme disagree by 15%!
So, go measure out several different volume amounts of real black powder with your favorite measure. Then weigh each of those piles and plot the results: grains by weight vs grains by volume. That's called a calibration curve, and it will tell you what the error in your measuring device is. And that's what it is - error in the measuring device, not some strange property of the powder.
If you are getting 80 grains by weight (of real black powder) for 100 grains by volume out of your powder measure, then your measure has a 20% error and should be allowed the decency of an early retirement. In a very deep lake. And you are shooting 80 grains, not 100 grains.
This leads to an answer to the question, "What size increment?" 2 grains by volume, measured by an accurate measure, is about the smallest that you can accurately dispense. This would also produce the smallest increments in POI groups that one can realistically get shooting off a rest. Depending on the shooter's ability, 5 grains might be a better choice. 5 grains is also the largest increment that I'd recommend.
And it doesn't matter whether 2f or 3f, unless you intend to shoot both in the same gun, in which case you need to work up a load for each granulation; the results will be different.