How do I convert RPM to RCF?

I need to convert 3,000 RPM in a Beckman centrifuge to g (or also RCF). I’m a little confused as to how to do this exactly though, despite having found some equations. Supposedly it’s based on the radius of the rotor and then other stuff says it depends on the length between the rotor and the sample. But I’m confused as to how to measure it out. Do I measure out the length between the rotor and the edge of the sample, or the rotor and the REAR edge of the sample? I hope you understand what I mean.