0 cap holds about 500mg. For simplicity sake I would probably make the caps 250 active, but for precision you will have to do a few tests to find a filler that weighs the same by volume or do a little math. Really it depends on what you're capping. If one is very fine versus one being very chunky you could wind up a fairly large difference.
But to answer your actual question you would need 23 parts active to 25 filler assuming you got the mix correct. Which is why I would just go 250mg for a 1:1 ratio. How you cap and pack will make a difference as well as what you're capping. Powdered plant or fungal material will be way less dense than something like bulk supplements.
Assuming you you get all that correct and have an equal filler you will want to run a batch of filler only and make sure you are actually getting the MG/cap you want total.
kind of like DJ said the key is the filler and the mixing