We tried using creatine but it was a mess, didn't fill the caps all the way, therefore inconsistent.
My husband likes using corn starch - fills better. If you're not careful, it's a huge mess.
You need to select your filler to match what you are dosing if you want consistent strength caps. It needs to be similar for both particle size and density. If you don't achieve a homogeneous mixture you will get seperation and inconsistent caps.
IDK what you're capping. Take a sample of what you're capping and put it next to the filler you're thinking of using. Are the particle sizes about the same? Measure out a Given volume of each, say 1/8 teaspoon, weigh each. are the weights the same or close? If you can answer yes to both you have a good filler for that substance. If you're capping something that comes as crystals and you fill with something fluffy like protein powder as you're capping the crystals will settle to the bottom of you're mixture. You're fist caps will have more powder and the last ones will have more crystals.