I don't know what the rules are on lemmy.ml, but some instances let anybody create a new community. There's no guide or doc that I know of, but it's as easy as making a post is. I just looked for a calculator community and couldn't find one, so I made it. If lemmy.ml won't let you make a community, either ask the mods there to approve one, or sign up on an instance that isn't as locked down. You can even make your own instance and still federate with the rest of the Lemmy-sphere.