I've monkeyed around a bit with capping. Got a capping kit, caps, and scale for a good price a few months ago. Thanks NP!!
What you need to do is figure out what it is you want to cap, how much is a dose, and do some math to figure out what size you need. Under the caps on NP, there's a description of approximately how much each will hold. If you have 800mg that you want to cap, and there's one that holds up to 750mg, you can either get a size that'll fit about half the dose and just partially fill the pills. If there's extra room and you are paranoid about that, then put in a filler like rice flour.
A word to the wise, though... Not all products have the same density. 25mg of one thing may take the space of 15mg of something else, or 40mg of something else. It all depends on the specific bulk product that you are trying to cap. Though the bulk caps that NP sells (in bags of 1000) will take a bit of pressure, it may drive you nuts trying to gently and patiently put gradual pressure on the tamper to compact it enough.
If you tinker a bit and get used to it, it can be fun and fruitful "hobby"!!

I'd start out capping something simple and inexpensive at first, like an amino or something, just to get used to doing it and measuring right. Don't start with something that has an aptitude for being overdose, lest your math be wrong! :lol:
Good luck!