Have any you made your own lens adapter? Or do any of you know a lot about flange focal distance?
My mirrorless camera has a flange focal distance of 18mm and the system I'm trying to adapt has ffd of  40mm. Do I just separate the mount from the body by 22mm? Or are there other factors/measurements to take into account?

The best thing is to just buy a commercially-available adapter. What camera and what lens mount are you trying to mate?

I purchased a Fotodiox adapter.

What system/lens are you trying to adapt? If it's anything remotely common you can probably buy an adapter for less than your time is worth to make one.